Fix stop reading message not showing up

sohstationviewer/view/main_window.py

@@ -712,6 +712,7 @@ class MainWindow(QtWidgets.QMainWindow, UIMainWindow):
 
         try:
             del self.data_object
+            self.data_object = None
             self.processing_log = []
             self.clear_actions_from_forms_menu()
         except AttributeError:
@@ -928,6 +929,9 @@ class MainWindow(QtWidgets.QMainWindow, UIMainWindow):
         Plot using data from self.data_object with the current options set
             from GUI
         """
+        if self.data_object is None:
+            return
+
         plotting_start_msg = 'Start plotting data...'
         display_tracking_info(self.tracking_info_text_browser,
                               plotting_start_msg)