Fix 500 error when loading dashboards with slice having deleted datas…

…et (#12535)

superset/models/dashboard.py

@@ -237,6 +237,7 @@ def full_data(self) -> Dict[str, Any]:
                 # Filter out unneeded fields from the datasource payload
                 datasource.uid: datasource.data_for_slices(slices)
                 for datasource, slices in datasource_slices.items()
+                if datasource
             },
         }