Public variable and Runtime error 91: Object Variable or With block variable not set

Hi all, I have a userform with a textbox that I load from a standard module that I use to edit cell comments more easily than Excel's comment boxes. Everything works (ie. changing cells and the textbox updating to the new cell's comment, automatic storing of the textbox data to the previous cell's comment. What doesn't work is when a cell has no comment, and I add one to the textbox and then click on a different cell, I get thrown a Object variable or with block not set on the line bolded below.

Also, in a standard module elsewhere, I set the public variable PrevCellRange to the active cell upon load of the userform as follows:

       PrevCellRange = Application.ActiveCell.Address

Then in my worksheet module, I have the following main code, with public variables set atop the module:

Public PrevCellRange As String
Public NewCell As String

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Application.ScreenUpdating = False

    PrevCellRange = NewCell
    NewCell = ActiveCell.Address

    If myUserForm.myTextBox <> vbNullString And Range("userFormIsLoaded") = 1 Then 
        If Range(PrevCellRange).Comment.Text <> vbNullString Then
            Range(PrevCellRange).Comment.Delete    
        End If
        Range(PrevCellRange).AddComment.Text Text:=myUserForm.myTextBox.Text  
    End If

'Refresh Text Box with Cell Change
    If Application.ActiveCell.Comment Is Nothing Then
        myUserForm.myTextBox.Text = ""
    Else
        myUserForm.myTextBox.Text = Application.ActiveCell.Comment.Text
    End If

Thanks all for your help again, this is causing me to pull my hair out. Frank