Other check for null pointer before calling decref

src/python/qgspythonutilsimpl.cpp

@@ -580,11 +580,13 @@ bool QgsPythonUtilsImpl::evalString( const QString& command, QString& result )
   bool success = nullptr != res;
 
   // TODO: error handling
+  if ( res )
+  {
+    if ( success )
+      result = PyObjectToQString( res );
 
-  if ( success )
-    result = PyObjectToQString( res );
-
-  Py_XDECREF( res );
+    Py_XDECREF( res );
+  }
 
   // we are done calling python API, release global interpreter lock
   PyGILState_Release( gstate );