How to set value of label and textarea when a button is click?

Buoy
2
  1. the error message says

/Users/npalardy/Desktop/Buoy/For Norman/Norm Work In Progress/junktest.bui:31:0: error: class ‘GlobalVars’ has no shared member, constant, nested enum, or nested structure ‘x_MainWindow’

so make x_mainWindow SHARED PUBLIC
In your code you never create an instance of Globals but I suspect the intent is, or was, to create a single shared set of global variables
SHARED PUBLIC means that you do not need to create an instance of the GlobalsVars class for the shared property to exist

  1. once you do step 1 you will likely get this as an error

/Users/npalardy/Desktop/Buoy/For Norman/Norm Work In Progress/junktest.bui:41:0: error: class ‘btn_ChangeAddress’ has no field ‘target_window’

which is true there’s no such property so comment out the line trying to set it
b.target_window = w ' ✅ Pass the window reference

3)next you’ll get
/Users/npalardy/Desktop/Buoy/For Norman/Norm Work In Progress/junktest.bui:22:0: error: undefined variable ‘Nothing’

because of this line
If GlobalVars.x_MainWindow IsNot Nothing Then

You CAN just use the ? operator so avoid the comparison entirely

GlobalVars.x_MainWindow?.lbl_Address.Text = "Your Address:"
GlobalVars.x_MainWindow?.ta_Address.Text = "Your Address"

see the example in optional_chain.bui

Optional-chain assignment: obj?.Field = value
Desugars to: If obj <> Nil Then obj.Field = value

The assignment is silently skipped when the receiver is Nil.

after these changes this turns into

Public Window MainWindow
    Title = "MainWindow"
    Frame = (200, 200, 640, 480)

    Control Label lbl_Address
        Text = "Label"
        Frame = (16, 16, 120, 20)
    End Control

    Control TextArea ta_Address
        Text = "TextArea"
        Frame = (136, 16, 300, 200)
    End Control
End Window

Public Class GlobalVars
    Shared Public Property x_MainWindow As MainWindow
End Class

Class btn_ChangeAddress Inherits Button
    Handle Action()
            GlobalVars.x_MainWindow?.lbl_Address.Text = "Your Address:"
            GlobalVars.x_MainWindow?.ta_Address.Text = "Your Address"
    End Handle
End Class

Sub Main()
    Dim w As New MainWindow()
    GlobalVars.x_MainWindow = w 

    ' Set initial values
    w.lbl_Address.Text = "My Address:"
    w.ta_Address.Text = "My Address"

    ' Create button and capture window reference
    Dim b As New btn_ChangeAddress()
    b.Caption = "Change to Address"
    b.Move(136, 224, 300, 32)
    w.AddSubview(b)

    w.Show()
    Application.Run()
End Sub