Mathmatica 12.0 crashed when exporting a graph

My mathematica crashes when I simply print a graph to a PDF. For example, when I input this:

Graph[{0->1}] 

and then export its output to a PDF, the notebook crashes.

Most puzzling of all, when I input this:

Graph[{0->1}, PlotTheme->"IndexLabeled"] 

the output can be normally exported.

Can anyone give me a clue?