Steps To Reproduce:
1. Open new project.
2. Click on the play button.
3. Click- Ctr+Shift+P
4. Click on Step Btn
5. Observe the Console for the Assertion failed error.

Note: This error will not come if click on the pause button and then click on the step button.

Work Around: Pause the game by using pause Btn instead of using hotkeys: Ctr+shift+P.

Actual Result: Editor giving Assertion failed error when the user clicks on step button in the pause state.

Expected Result: In Pause state, if the user clicks on the step button it should not give any error.

Reproduced on: 2017.3.0b9,2017.3.0b1, 2017.2.0f3, 2017.1.2f1

License Type - Free
Platform - Windows

Stack Trace: Assertion failed: Assertion failed on expression: '!dest.m_MultiFrameGUIState.m_NamedKeyControlList'
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)