Could be a simple fix.

When sharing from another app, ei. Gallery, to Textra a floating window is brought up.

If you want to back out and close the window by using the upper-right context, menu the first selection is 'Block', where the other Textra floating windows will list 'Done'.

This makes it easy to accidentally block numbers. The solution that comes to mind is to keep the context menu consistent and keep 'Done' as the first option in any floating windows. Put Block to the bottom.