Remove annotation causes Xreader to crash

 * Xreader version 2.8.3
 * Distribution - Mint 20.1 Cinnamon

Issue
Xreader crashes if you have the annotation reference selected in the Annotations pane, then try to remove the annotation in the text using the context menu.

Steps to reproduce

1. Open the attached pdf document. annotations_bug.pdf
2. Open the sidebar (F9), then select "Annotations". Click on the page number to get the annotations dropdown, then click on the annotation reference.
3. Use the scroll wheel if necessary to bring the annotation into view. Right-click on it and select "Remove annotation"

RESULT: The App. crashes.

This only happens if you have the annotation reference highlighted in the side pane. If the focus is elsewhere, then the annotation is removed as expected.

Expected behaviour

The annotation should be deleted.

Other information
This bug was originally picked up in "Add remove annotation functionnality" #181.

Start by reproducing the crash with the attached annotations_bug.pdf in Xreader 2.8.3: open the Annotations pane, select the annotation reference, and choose “Remove annotation” from the context menu. Trace the annotation-removal entry point reached from that context menu and verify that the annotation is deleted without crashing when the reference remains selected.