Reusable Blocks respond erratically when external media files are dragged & dropped into them

[mrfoxtalbot]

Describe the bug
It is currently possible to drag and drop a file from your computer into an existing reusable block. From the editor, it will look like the file was added, but it will not be saved and in some instances it will make the editor crash.

To reproduce
Steps to reproduce the behavior:

1. Create a new post, add a reusable block that has at least two blocks in it.
2. Drag & drop an image from your computer inside the reusable block
3. Click on preview > The newly added image will not be inside the reusable block
4. Click on "edit reusable block" > The reusable block will become editable but the image will disappear

Expected behavior
The drag & drop action should either edit the reusable block directly (then add the image to it and change its state to "being edited") or prevent the user from dropping a file inside at all.

Screenshots
With the Core version of Gutenberg (5.4.1) the image will just disappear if you reload the editor, try to preview it or edit the reusable block:

With the plugin version of Gutenberg (8.1.0) this action will cause the editor to crash:

Editor version

• WordPress version: 5.4.1
• The error occurs with and without the plugin, but varies slightly (see above)
• If the Gutenberg plugin is installed, which version is it? 8.1.0

Desktop
MacOS 10.13.6 (17G2307)
Google Chrome 79.0.3945.88