Fix type error when merging blocks

When we merge two blocks by pressing the BACKSPACE KEY, the TinyMCE
instance belonging to the second block is destroyed. This results in a
type error raised by a TinyMCE event handler that is listening to the
*same* event.

By *immediately* preventing propagation, we can stop TinyMCE's handler
from ever running.

blocks/rich-text/index.js

@@ -550,6 +550,11 @@ export default class RichText extends Component {
 			}
 
 			event.preventDefault();
+
+			// Calling onMerge() or onRemove() will destroy the editor, so it's important
+			// that we stop other handlers (e.g. ones registered by TinyMCE) from
+			// also handling this event.
+			event.stopImmediatePropagation();
 		}
 
 		// If we click shift+Enter on inline RichTexts, we avoid creating two contenteditables