check if self.engine exists before calling dispose in close()

Author: adamhadani

rdflib_sqlalchemy/store.py

@@ -253,7 +253,8 @@ def close(self, commit_pending_transaction=False):
         Close the current store engine connection if one is open.
 
         """
-        self.engine.dispose()
+        if self.engine:
+            self.engine.dispose()
         self.engine = None
 
     def destroy(self, configuration):