Fix 500 error when loading dashboards with slice having deleted dataset

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
  },
  }