Back button can cause pseudocode view to close

* Open angr-management
* Select pseudocode tab
* Click Back navigation button (left-pointing green arrow at bottom of pseudocode view)
* Tab closes for some reason

Expected:
* Do nothing if there is nothing to go back to

Edit: After looking at the code, this is behaving as written and not some random bug, however in my opinion this behavior is surprising and it was reported to me by a user who was also surprised, and this is inconsistent with other view behavior such as the Disassembly tab's navigation buttons.

I do see an argument for quickly pressing F5 for decomp then pressing escape to return to disassembly (although that does not work as written), but I think instead it should simply focus the disassembly instead of closing the view if there is no previous history, or perhaps do nothing as the disassembly view does.