VB.net WindowsForm changing then [System.Data.MissingPrimaryKeyException] - vb.net

Question

-I created 2 forms after I use hide() and show() to switch between form.
-In the 2nd form, I create datatable (bond to datagrid) and primary key for datatable to use find().
-After I press [BACK] button form 2nd form to 1st form (use hide() and show() function) and come back to use 2nd form application again. the runtime error show this [System.Data.Missing.PrimaryKeyException: Table doesn't have a primary key]
1st Form
Public Class MainMenu
Public MainForm As MainMenu
Public AssetCheckForm As AssetCheck
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
AssetCheckForm.Show()
MainForm.Hide()
End Sub
Public Sub MainMenu_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
MainForm = New MainMenu()
AssetCheckForm = New AssetCheck()
End Sub End Class
2nd Form
Dim dtAsset As New DataTable("AssetTable")
Public Sub readData()
If readStatus = 0 Then
Try
Dim splits As String()
Using sr As StreamReader = New StreamReader(inputcsvname)
'read the first line for the table columns
splits = sr.ReadLine.Split(","c)
For i As Integer = 0 To UBound(splits)
dtAsset.Columns.Add(splits(i))
Next
'read the rest of the lines to add rows
Do While Not sr.EndOfStream
splits = sr.ReadLine.Split(","c)
dtAsset.Rows.Add(splits)
Loop
End Using
Catch ex As Exception
Finally
End Try
dtAsset_display = dtAsset.Copy()
totalcount.Text = getRowsCount(dtAsset_display)
dtAsset.Columns("AsstCode").Unique = True
dtAsset.PrimaryKey = New DataColumn() {dtAsset.Columns("AsstCode")}
'bind display part to DataGrid
DataGrid1.DataSource = dtAsset
Private Sub BackButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonF4.Click
clearData()
MainMenu.MainForm.Show()
MainMenu.AssetCheckForm.Hide()
End Sub
I think I missed something about VB.net WinForm concept.
Does anyone know what this error means?

Answer 1

The fact that you are calling MainForm = New MainMenu() in MainForm it means you now have two instances of MainForm.
So then when you call MainMenu.MainForm.Show() you are showing your new instance, not the existing one.
You need to change this around so that you store the reference to the existing MainForm in your AssetCheckForm form.
Something like this:
Public Class MainMenu
Private AssetCheckForm As AssetCheckForm
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Me.AssetCheckForm = New AssetCheckForm
Me.AssetCheckForm.MainMenu = Me
Me.Hide()
Me.AssetCheckForm.Show()
End Sub
End Class
Public Class AssetCheckForm
Friend MainMenu As MainMenu
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Me.MainMenu.Show()
Me.Close()
End Sub
End Class