Assert on removing annotations object when sequence object was added

To reproduce the issue:

1. Open a nucleotide FASTA sequence
2. Create a new annotation object and document (e.g. search for repeats in the sequence)
3. On the annotations document select "Add > Add object to document"
4. Select the sequence object in the appeared dialog => The document must now contain the sequence and the annotations object
5. Right-click on the annotations object and click "Delete" keyboard button

Result: a debug error occurs.