Cannot get system fields with getFieldByName behaviours method

To reproduce:

• Set up a behaviour on a system field (e.g. Priority). Doesn't matter whether it does anything or not - it just needs to include the field.
• Call setAllowInlineEdit on the field in the initialiser, e.g.:

  getFieldByName("Priority").setAllowInlineEdit()
  

• Go to an issue where the behaviour applies and click on the field on the view screen.

Expected result: Field turns into an inline edit box.

Actual result: The edit screen pops up.

This works correctly with custom fields.