Steps to reproduce

1.  go to https://demoauthor.magnolia-cms.com/
2.  go to Resource Files app
3. open /travel-demo/dialogs/pages/pageProperties.yaml for editing
4. remove all fields from the first tab so you are left with the following:
   form:
     tabs:
       - name: tabMain
         fields:
5. Open app launcher menu
6. Open Definitions app
7. See NPE

Expected results

An empty value for "fields" should not affect the definitions. This should only result in a minor error. It should not cause the definitions app to break : 

https://documentation.magnolia-cms.com/display/DOCS62/YAML#YAML-Syntax

Actual results

The definitions app breaks when you try to access it while having a dialog containing an empty YAML "fields" value.

Workaround

Check each dialog to see which have empty value for "fields".  

Development notes

Please see linked tickets for possible reasons this is happening.