vb.net checking process is ended, and modify the UI

I have a problem, my program (when launched the process) minized into system tray, and when the process ended, it shows in normal state, but its not working with my solution, i get this error message:

An unhandled exception of type 'System.InvalidOperationException' occurred in System.Windows.Forms.dll

My code:

Dim p as new Process
p.filename = "somefile.exe"
p.workingdirectory = "somepath"
p.EnableRaisingEvents = True
AddHandler p.Exited, AddressOf Main.Main_Process_Closed
p.Start()

The handler in the main form:

Public Sub Main_Process_Closed(ByVal sender As Object, ByVal e As System.EventArgs)
        Me.Visible = True
        Me.WindowState = FormWindowState.Normal
        NotifyIcon1.Visible = False
    End Sub

But what is the problem? Thanks for help.