Fix showing error dialog when calling vsimple_error_message_box()

SimpleDialog requires a parent, when none is provided (as there is no parent available during startup), the error message is queued and displayed after MainWindow is available.
Calling SimpleDialog::exec() directly returns if no parent is set (hence the dialog is not shown)

This resulted in silently ignoring error messages that were shown using vsimple_error_message_box(), as this function did not provide a parent (and the queue messages were not dequeued anymore as the MainWindow was already available)

Change-Id: I2f4c1b4926c196ab40b035161cec530105396537
Reviewed-on: https://code.wireshark.org/review/5519
Reviewed-by: Michael Mann <mmann78@netscape.net>