1.10.2 – Image upload not working for markitup fields

In the latest update of Statamic (1.10.2), the image upload button in markitup fields isn't working anymore (clicking on it does nothing). I don't see any errors in the console, so I'm not sure what's happening. At first I thought it may have something to do with using a markitup field in a replicator set, so then I moved it outside and it still won't work.

Here's my field definition:

fields:

  test_markitup_field:
    display: TEST FIELD
    type: markitup
    image_dir: assets/img/