Feature Backlog - Unassigned Backlog panel does not display if more than 4 PIs are selected

Issue Summary

On Backlog> Features, The list of Feature display.
If one hit the button "Unassigned Backlog" a panel will appear on the right side, and display the list of unassigned items.

Note- Unassigned backlog only appears if at least one PI is selected

This works normally, unless the number of selected PIs is more than 4. In this case, the panel keep spinning and will not display the expected result

Steps to Reproduce

1. Go to Backlog>Feature
2. Select a program and at least 4 PIs in the top tier filter
3. Hit the button "Unassigned backlog" and notice the list displays straight away
4. Change the top tier filter and add another PI (Total=5PIs selected now)
5. Hit the button "Unassigned backlog and notice that the list does not display anymore (500 error in console)

Expected Results

The list should display for more than 4PIs selected 

Actual Results

If more than 4 PIs are selected, the panel open to a blank page and keep loading
example below

backlog-PI 4-5.mov

Workaround

Use the Unassigned Backlog section below list of all PIs