Solution Import - Failed deleting solution. The object you tried to delete is associated with another object and cannot be deleted.

[TomProkop]

Error

Failed deleting solution. The object you tried to delete is associated with another object and cannot be deleted.

Cascade Delete failed due to cascade restrict relation.

Solution

Trying to do this in classic solution viewer, we get the opportunity to download an error log.

Here you can see that Theme is referencing a webresource (logo image) from the app. Remove the dependency and you are good to go.