[Crash] ColumnView crashes when going back while loading large folder

### What Happened?

Opened a folder in ColumnView and the clicked on a subfolder containing a very large number of files. Before all the files were loaded, clicked on another subfolder contained many files. After a pause Files crashed with the message
`ERROR:../src/View/AbstractDirectoryView.vala:2744:files_abstract_directory_view_schedule_thumbnail_color_tag_timeout: assertion failed: (slot is Files.AbstractSlot && slot.directory != null)
Bail out! ERROR:../src/View/AbstractDirectoryView.vala:2744:files_abstract_directory_view_schedule_thumbnail_color_tag_timeout: assertion failed: (slot is Files.AbstractSlot && slot.directory != null)
Aborted`

This may not happen every time but is likely to happen with large folders

### Steps to Reproduce

1. Open ColumnView
2. Click on a large subfolder
3. Click on another large subfolder

### Expected Behavior

The loading folder should be cancelled and the second one begin to load without any crashes

### OS Version

7.x (Horus)

### Software Version

Latest release (I have run all updates)

### Log Output

_No response_

### Hardware Info

_No response_