Communicating between forms in VB.net

I have a main form listening for communication on a COM port and another form with a button. The button opens a save file dialog box which, on successful return of the ShowDialog() method, sets a public string "FilePath" to the .FileName property and then writes a command to the serial port. The hardware listening on the other end sees this command and begins sending data. My main form sees this data and branches to a sub to collect it and save it. In this sub I try to access the "FilePath" public string from the other form to use as the save path for the data coming in but it is always "Nothing". The form has not closed yet, and I KNOW that that variable has been assigned in the button click handler...

What is going on?

Public Class frmDebug

    Public FilePath As String

    Private Sub Button_DumpDataflash_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button_DumpDataflash.Click

        SaveFileDialog1.Filter = "Binary File|*.bin"
        SaveFileDialog1.InitialDirectory = System.Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)

        If Not SaveFileDialog1.ShowDialog() = Windows.Forms.DialogResult.OK Then
            Exit Sub
        End If

        FilePath = SaveFileDialog1.FileName

        frmMain.SerialPort1.Write(New Byte() {60}, 0, 1)

    End Sub

End Class