BitBucket import stuck/hangs indefinitely

Summary

Something is causing imports from BitBucket to hang indefinitely.

Steps to reproduce

Import a project

Expected behavior

Import successful or fail

Actual behavior

Import is stuck

Relevant logs and/or screenshots

irb(main):004:0> p.import_error 
=> "{\"message\":\"The remote data could not be fully imported.\",\"errors\":[{\"type\":\"pull_request\",\"iid\":1,\"errors\":\"undefined method `fetch' for nil:NilClass\"},{\"type\":\"pull_request\",\"iid\":2,\"errors\":\"undefined method `fetch' for nil:NilClass\"}]}" 

Results of GitLab environment info

GitLab 8.15

https://gitlab.zendesk.com/agent/tickets/58060