I'm trying to trace the bitmap of an image so I can change a face's eye color on a printable pdf (I'm using it to make an origami doll for my best friend, and I'm trying to match her eye color). I succeeded once before but somehow deleted the file. I'm trying again, but each time I select the object and click "trace bitmap", no preview shows up in the window. Whenever I click "update preview" or "start", it'll say that no object is selected. I've attached screenshots of what's been happening. I've checked to make sure I've that it's the current version, but beyond that I have no clue what to do. If someone could please give me some advice as to how to rectify this issue, that would be great. 

PDFs may contain vectors and/or bitmaps. I'd try the Arrange>Deep-ungroup extension and see if the eyes are selectable.

Best to save and use SVG for editing, and PDF for final output.

Thank you, but when I try to deep ungroup, I get an error message, and Inkscape crashes 

That's not expected. 

Please share an example Inkscape file for evaluation.

Here it is. 

Well, the ungroup extension seems to have some issue. 

Fortunately, the objects can be edited by double clicking a few times to get into the groups, or by clicking the ungroup button a few times to ungroup all objects.

All objects are paths, so no need to trace.