Handle missing custom blocks gracefully in RichEditor (v4)

[SeriousKen]

When content contains a custom block but that block is not registered with the RichEditor field, the field shows a block with an edit and a delete button, but clicking the edit button causes an internal error. I feel there should be a more graceful way of handling this case such as hiding the block from the editor or being able to define a fallback block. I can see a situation where a package provides a block, but then this is uninstalled, leaving the reference in the content.