Crash when cancelling db connection dialog

Fixed a crash when the Dialog to change the database connection was cancelled.

ArtNet/gui/windows/importer_window.py

@@ -947,7 +947,7 @@ class ImporterWindow(ArtnetMainWindow):
         dialog.ui.port_line_edit.setText(str(prev_db_data["port"]))
 
         db_data: dict = dialog.exec_()
-        if len(db_data.keys()) == 0:
+        if db_data is None or len(db_data.keys()) == 0:
             return
         try:
             self.main.change_db_connection(host=db_data["host"], port=db_data["port"],