To reproduce: Create collection using

db.coll.insertMany([ 
  { subdoc: { a: { b: 1 } } }, 
  { subdoc: null }
]);

then sample this collection to create a diagram and click `+` to add a field to `subdoc.a`.

This results in an edit that fails to apply to the determined schema, since `addFieldToJSONSchema` expects a single schema and not an optional field represented via `anyOf`.

Because the edit fails to apply, no data modeling screen continues to work, and the only way to get out of this "soft lock" is to remove the offending data modeling file from disk and then restart Compass.