Validate form for presence of required fields

Make following fields compulsory: entity_type, entity and view mode.
If any error occurs, show the error in the dialog before proceeding further.

At the final step before submit happens, validate form to ensure all the required fields are present (from previous step as well as this step)

Validating presence of fields from previous step just to be sure, a conservative step.

d.o issue: https://drupal.org/node/2279175