Cannot view task pane Impress

I accidentally closed the task pane in Impress, and cannot open it again.

Initially the option “task pane” did not appear in my “View” menu, so I could not activate it. I went to “Tools - Customise” and added the option “task pane” to the “view” menu. I also added a keyboard shortcut for “Task pane” (F12). After this, the option “Task pane F12” appeared in my “view” menu.

However, when I select “task pane” in the menu, no checkmark appears beside it, and the task pane does not appear. If I press F12, the task pane does not appear. If I close and re-open LibreOffice, it still doesn’t work.

How might I make the task pane re-appear?

I am running, Build ID 1:5.0.3~rc2-0ubuntu1~trusty2, on Linux Mint 17.3 Cinnamon 64-bit