Changing Focus of Field Displays Error in JIRA Agile

Steps to Reproduce:

1. Install JIRA 6.3.15;
2. Install JIRA Agile 6.6.60;
3. Create a Custom Field of type Text Field (multi-line);
4. Add it to the Default Screen;
5. Create a Project and a board;
6. Change the Renderer of this field in this project to Wiki Markup;
7. Navigate to the board and create an Epic;
8. Create an issue (from anywhere inside JIRA Agile)
9. Add something to the new field and change focus.

Expected Behavior:

JIRA would change the focus to the next field accordingly.

Actual Behavior:

JIRA shows an error as shown in the video attached (error.mov). There is also a backup attached that can be used to replicate this (account/password: matheus/matheus).

This issue is also happening for Custom User Pickers (Single and Multi)