Update dash_wrapper.py (#511)

* Update dash_wrapper.py

fixed error message

* Update models.py

type hint

* Update dash_wrapper.py

Author: nicolasstelter

django_plotly_dash/models.py

@@ -58,7 +58,7 @@ def save(self, *args, **kwargs): # pylint: disable=arguments-differ
                 self.slug = self.slug + str(exist_count+1)
         return super().save(*args, **kwargs)
 
-    def as_dash_app(self):
+    def as_dash_app(self) -> DjangoDash:
         '''
         Return a DjangoDash instance of the dash application
         '''