ComboBox.SelectedItem giving Null value

I have been stuck in this for whole day. I am creating a UI using PowerShell forms. In that:

user select an option from 1st combobox. click button Go
Based on the selection, a panel will appear having another combobox.
if user select another option in 1st combobox then another panel appears with another combobox
After selecting options from panel comboboxes, user clicks on start button.
This leads to a function which stores the selected options to a variable.

Problem

Now when user selects the options from the comboboxes of panels, I am using $combobox.selecteditem.Tostring to get the values. But it gives me NULL result. Here is my code..

$global:Button1Clicked = 0;
Add-Type -AssemblyName System.Windows.Forms
Add-Type -AssemblyName System.Drawing
[System.Windows.Forms.Application]::EnableVisualStyles()
[System.Windows.Forms.FormClosingEventHandler]

$Form = New-Object system.Windows.Forms.Form
$Form.ClientSize = '1500,800'
$Form.text = "NextUI "
$Form.BackColor = 'White'
$Form.TopMost = $true
$Form.add_closing(
{
if($global:Button2Clicked)
{
$global:Button2Clicked = 0;

$_.cancel = $true;
}
})


$panel1 = New-Object system.Windows.Forms.Panel
$panel1.AutoSize = $true
$panel1.Width = 1200
$panel1.Height = 200
$panel1.location = New-Object System.Drawing.Point(50,100)
$panel1.Visible = $false
$panel1.Controls.Add($label3)
$panel1.Controls.Add($comboBox2)
$panel1.BorderStyle = 1


$panel2 = New-Object system.Windows.Forms.Panel
$panel2.AutoSize = $true
$panel2.Width = 1200
$panel2.Height =200
$panel2.location = New-Object system.Drawing.Point(50,100)
$panel2.Visible = $false
$panel2.Controls.Add($label4)
$panel2.Controls.Add($ComboBox3)
$panel2.BorderStyle = 1

$Label1 = New-Object system.Windows.Forms.Label
$Label1.text = "Select Module"
$Label1.AutoSize = $true
$Label1.location = New-Object System.Drawing.Point(35,50)
$Label1.Font = 'segoe ui,9.5'

$Label2 = New-Object system.Windows.Forms.Label
$Label2.text = "SharePoint Settings"
$Label2.AutoSize = $true
$Label2.location = New-Object System.Drawing.Point(35,15)
$Label2.Font = 'Segoe UI Semibold,9.5'

$Label3 = New-Object system.Windows.Forms.Label
$Label3.text = "Choose file and folder permission option"
$Label3.AutoSize = $true
$Label3.location = New-Object System.Drawing.Point(100,200)
$Label3.Font = 'segoe ui,9.5'


$Label4 = New-Object system.Windows.Forms.Label
$Label4.AutoSize = $True
$Label4.text = "File Permission"
$Label4.location = New-Object System.Drawing.Point(100,200)
$Label4.Font = 'Segoe UI ,9.5'


################ Module combo box ################

$ComboBox1 = New-Object system.Windows.Forms.ComboBox
$ComboBox1.text = "Select Module"
$ComboBox1.width = 200
$ComboBox1.height = 20
$ComboBox1.location = New-Object System.Drawing.Point(310, 45)
$ComboBox1.Font = 'Microsoft Sans Serif,10'
$combobox1.items.Add("ControlSettings")
$combobox1.items.Add("NextSettings")


######## file and folder permission #############
$ComboBox2 = New-Object system.Windows.Forms.ComboBox
$ComboBox2.text = "select an option"
$ComboBox2.width = 200
$ComboBox2.height = 20
$ComboBox2.location = New-Object System.Drawing.Point(450, 200)
$ComboBox2.Font = 'Microsoft Sans Serif,10'
$ComboBox2.items.Add("View") 
$ComboBox2.items.Add("Edit") 

$ComboBox3 = New-Object system.Windows.Forms.ComboBox
$ComboBox3.text = "select an option"
$ComboBox3.width = 200
$ComboBox3.height = 20
$ComboBox3.location = New-Object System.Drawing.Point(450, 200)
$ComboBox3.Font = 'Microsoft Sans Serif,10'
$ComboBox3.items.Add("View") 
$ComboBox3.items.Add("Edit")


Function Button2_Click()
{

if ($ComboBox1.SelectedIndex -eq 0)
{
$panel2.Visible = $true
$panel1.Visible = $false
}

if ($ComboBox1.SelectedIndex -eq 1)
{
$panel1.Visible = $true
$panel2.Visible = $false

}


}

$Button1 = New-Object system.Windows.Forms.Button
$Button1.text = "Start"
$Button1.width = 150
$Button1.height = 30
$Button1.BackColor = '#F6CEE3'
$Button1.DialogResult = [System.Windows.Forms.DialogResult]::OK
$Button1.location = New-Object System.Drawing.Point(500, 700)
$Button1.Font = 'segoe ui,10'
$Button1.Add_Click({
Button1_Click;
$global:Button1Clicked = 1;
})

############# Button 'Go' #############

$Button2 = New-Object system.Windows.Forms.Button
$Button2.text = "Go"
$Button2.width = 100
$Button2.height = 30
$Button2.BackColor = '#F6CEE3'
$Button2.DialogResult = [System.Windows.Forms.DialogResult]::OK
$Button2.location = New-Object System.Drawing.Point(680, 43)
$Button2.Font = 'segoe ui,10'
$Button2.Add_Click({
Button2_Click;
$global:Button2Clicked = 1;
})


######### code Starts ###########

function Button1_Click()
{
    $link = $comboBox2.SelectedItem.ToString();
    $linktype = $comboBox3.SelectedItem.ToString();


}
####### end of code ######

$form.Controls.Add($Panel1)
$form.Controls.Add($Panel2)
$form.Controls.Add($button1)
$form.Controls.Add($comboBox1)
$form.Controls.Add($button2)
$form.Controls.Add($pictureBox1)
$form.Controls.Add($label1)
$form.Controls.Add($label2)

[void]$Form.Add_Shown({ $Form.Activate() })
[void]$Form.ShowDialog()

Solution

The issue is that your variables are being set in a function, so they are scoped to that function. There's really no reason to put that in a function, put it directly in the .add_click() instead. Or if you feel that you need to keep it in the function you can scope the variable like you did with $global:ButtonClicked and set them to $global:Link and $global:LinkType.

Edit: Order does make a difference in PowerShell, so I did move some stuff around in your script to make it work right, but I was able to get values to reflect fine when I put them in the global scope.

$global:Button1Clicked = 0;
Add-Type -AssemblyName System.Windows.Forms
Add-Type -AssemblyName System.Drawing
[System.Windows.Forms.Application]::EnableVisualStyles()
[System.Windows.Forms.FormClosingEventHandler]

######### code Starts ###########

function Button1_Click()
{
    write-host "Link = $global:link"
    write-host "Link Type = $global:linktype"

}

Function Button2_Click()
{

if ($ComboBox1.SelectedIndex -eq 0)
{
$panel2.Visible = $true
$panel1.Visible = $false
}

if ($ComboBox1.SelectedIndex -eq 1)
{
$panel1.Visible = $true
$panel2.Visible = $false

}


}

####### end of code ######

$Form = New-Object system.Windows.Forms.Form
$Form.ClientSize = '1500,800'
$Form.text = "NextUI "
$Form.BackColor = 'White'
$Form.TopMost = $true
$Form.add_closing(
{
if($global:Button2Clicked)
{
$global:Button2Clicked = 0;

$_.cancel = $true;
}
})

################ Module combo box ################

$ComboBox1 = New-Object system.Windows.Forms.ComboBox
$ComboBox1.text = "Select Module"
$ComboBox1.width = 200
$ComboBox1.height = 20
$ComboBox1.location = New-Object System.Drawing.Point(310, 45)
$ComboBox1.Font = 'Microsoft Sans Serif,10'
$combobox1.items.Add("ControlSettings")
$combobox1.items.Add("NextSettings")


######## file and folder permission #############
$ComboBox2 = New-Object system.Windows.Forms.ComboBox
$ComboBox2.text = "select an option"
$ComboBox2.width = 200
$ComboBox2.height = 20
$ComboBox2.location = New-Object System.Drawing.Point(450, 200)
$ComboBox2.Font = 'Microsoft Sans Serif,10'
$ComboBox2.items.AddRange(@("View","Edit") )
#$ComboBox2.items.Add("Edit") 

$ComboBox3 = New-Object system.Windows.Forms.ComboBox
$ComboBox3.text = "select an option"
$ComboBox3.width = 200
$ComboBox3.height = 20
$ComboBox3.location = New-Object System.Drawing.Point(450, 200)
$ComboBox3.Font = 'Microsoft Sans Serif,10'
$ComboBox3.items.Add("View") 
$ComboBox3.items.Add("Edit")

$panel1 = New-Object system.Windows.Forms.Panel
$panel1.AutoSize = $true
$panel1.Width = 1200
$panel1.Height = 200
$panel1.location = New-Object System.Drawing.Point(50,100)
$panel1.Visible = $false
$panel1.Controls.Add($label3)
$panel1.Controls.Add($comboBox2)
$panel1.BorderStyle = 1


$panel2 = New-Object system.Windows.Forms.Panel
$panel2.AutoSize = $true
$panel2.Width = 1200
$panel2.Height =200
$panel2.location = New-Object system.Drawing.Point(50,100)
$panel2.Visible = $false
$panel2.Controls.Add($label4)
$panel2.Controls.Add($ComboBox3)
$panel2.BorderStyle = 1

$Label1 = New-Object system.Windows.Forms.Label
$Label1.text = "Select Module"
$Label1.AutoSize = $true
$Label1.location = New-Object System.Drawing.Point(35,50)
$Label1.Font = 'segoe ui,9.5'

$Label2 = New-Object system.Windows.Forms.Label
$Label2.text = "SharePoint Settings"
$Label2.AutoSize = $true
$Label2.location = New-Object System.Drawing.Point(35,15)
$Label2.Font = 'Segoe UI Semibold,9.5'

$Label3 = New-Object system.Windows.Forms.Label
$Label3.text = "Choose file and folder permission option"
$Label3.AutoSize = $true
$Label3.location = New-Object System.Drawing.Point(100,200)
$Label3.Font = 'segoe ui,9.5'


$Label4 = New-Object system.Windows.Forms.Label
$Label4.AutoSize = $True
$Label4.text = "File Permission"
$Label4.location = New-Object System.Drawing.Point(100,200)
$Label4.Font = 'Segoe UI ,9.5'


$Button1 = New-Object system.Windows.Forms.Button
$Button1.text = "Start"
$Button1.width = 150
$Button1.height = 30
$Button1.BackColor = '#F6CEE3'
$Button1.DialogResult = [System.Windows.Forms.DialogResult]::OK
$Button1.location = New-Object System.Drawing.Point(500, 700)
$Button1.Font = 'segoe ui,10'
$Button1.Add_Click({
$global:link = $comboBox2.SelectedItem.ToString();
$global:linktype = $comboBox3.SelectedItem.ToString();
Button1_Click;
$global:Button1Clicked = 1;
})

############# Button 'Go' #############

$Button2 = New-Object system.Windows.Forms.Button
$Button2.text = "Go"
$Button2.width = 100
$Button2.height = 30
$Button2.BackColor = '#F6CEE3'
$Button2.DialogResult = [System.Windows.Forms.DialogResult]::OK
$Button2.location = New-Object System.Drawing.Point(680, 43)
$Button2.Font = 'segoe ui,10'
$Button2.Add_Click({
Button2_Click;
$global:Button2Clicked = 1;
})

$form.Controls.Add($Panel1)
$form.Controls.Add($Panel2)
$form.Controls.Add($button1)
$form.Controls.Add($comboBox1)
$form.Controls.Add($button2)
$form.Controls.Add($pictureBox1)
$form.Controls.Add($label1)
$form.Controls.Add($label2)

[void]$Form.Add_Shown({ $Form.Activate() })
[void]$Form.ShowDialog()

When I ran that, set the two combobox values, and clicked Start I got text in the ISE saying what I set the Link and LinkType variables to, and was able to echo $global:link and $global:linktype to see the values correctly assigned.