Avoid infinite loop in quoteString

python/plugins/db_manager/db_plugins/connector.py

@@ -196,7 +196,7 @@ def quoteId(self, identifier):
     @classmethod
     def quoteString(self, txt):
         """ make the string safe - replace ' with '' """
-        if hasattr(txt, '__iter__'):
+        if hasattr(txt, '__iter__') and not isinstance(txt, str):
             txts = list()
             for i in txt:
                 if i is None: