1. What happened
While using Playmaker's Ecosystem browser (https://hutonggames.fogbugz.com/default.asp?W1181, https://github.com/jeanfabre/PlayMaker--Ecosystem--Browser), there's a failed assertion reported to console on first search:

Assertion failed: Assertion failed on expression: '!dest.m_MultiFrameGUIState.m_NamedKeyControlList'

This wasn't so in 5.6.

Note: there's another error appears, but that used to be there since as early as 5.3 (didn't check earlier):
ArgumentException: Getting control 2's position in a group with only 2 controls when doing Repaint
Aborting
UnityEngine.GUILayoutGroup.GetNext () (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/LayoutGroup.cs:115)
...

2. How we can reproduce it using the example you attached
- Open 'Playmaker > Addons > Ecosystem > Ecosystem Browser'
- Search for anything
---> The assertion