Add if statement in sql_handler to eliminate error message when trying to dispose an engine that is already closed

cdplib/db_handlers/SQLHandler.py

@@ -637,6 +637,7 @@ class SQLHandler:
 
     def dispose_engine(self):
         try:
-            self._engine.dispose()
+            if self._engine.closed:
+                self._engine.dispose()
         except Exception as e:
             self._log.log_and_raise_error(('An error occured when trying to dispose the SQL engine. Error: {}').format(e))