Deleting assigned tag breaks admin panel.

Steps:

1. Creat new tag under default Tags.
2. Edit e.g. Folder content type and add field "Taxonomy Entry Assignment".
3. Choose Tags in Taxonomy dropdown and save content type.
4. Publish new folder and assign tag created in step 1.
5. Delete this tag.
6. Try to visit Content structure or Dashboard.

Actual result: Error 404 or 500 occurs:
Content structure:

Could not find 'Content' with identifier 'array (
'id' => 155,
'languages' =>
array (
0 => 'eng-GB',
),
'versionNo' => NULL,
)'

Dashboard:

An exception has been thrown during the rendering of a template ("Taxonomy Entry with ID: '101' not found.").

Expected result: It is possible to access folder and assign new tag.