Excel UserForm.Show "Code execution has been interruped" error

I have a button on a worksheet that when clicked executes:

Private Sub cmdGetQuote_Click()
  frmSelectQuote.Show 
End Sub

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The first time the button is clicked, the user form shows.  I have a listbox come up and they click the item they want, then click the OK button, which goes through the list box and returns the selected value to the current worksheet.

Private Sub cmdOK_Click()
    On Error GoTo ErrHandler
    Dim lngItem As Long
    Dim I As Long
    Dim msg As String
 
    If ListBox1.ListIndex <> -1 Then
        For I = 0 To ListBox1.ColumnCount - 1
            msg = msg & ListBox1.Column(I) & vbCrLf
        Next I
    End If

    For lngItem = 0 To ListBox1.ListCount - 1
        If ListBox1.Selected(lngItem) Then
            With Sheets("Sheet1") '< qualify sheet here
                .Range("B2").Value = ListBox1.List(lngItem, 0)
            End With
        End If
    Next lngItem
    Unload Me
    
ExitHandler:
  Unload Me
  Exit Sub
    
ErrHandler:
  MsgBox "Error: " & Err.Number & " " & Err.Description & " :ListBox1_DblClick"
  GoTo ExitHandler
  Resume
End Sub

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I am then back at Sheet1 and see my value in cell B2.  Then the problem occurs.  I click on the button to show the form again and I get the Microsoft Visual Basic Error dialog box with the error:
Code execution has been interruped

If I click continue, everything is fine.  
Why am I getting this?
How do I prevent this from happening?