Fix cancel action: use thread quit() method instead of exit()

simple_etl.py

@@ -208,7 +208,7 @@ class SimpleETL:
             print("Input/Output layer is not valid")
 
     def on_pb_cancel(self):
-        self.thread.exit()
+        self.thread.quit()
         logger.info('ETL process cancelled')
 
     def on_pb_save(self):