Console throws NullReferenceException when user docks Animation/Animator Window in the editor, quits Unity and relaunches to create a new project.

Steps to reproduce:
1. Create > New Project
2. Window > Animation/Animator or Ctrl/Cmd + 6
3. Dock Animation/Animator Window anywhere
4. File > New Project...
5. Create new project or open any project
6. Observe errors on "Console".

Environment:
Occurring on both Windows and Mac

Occuring with :
Unity 2018.1.0b4 , 2018.1.0b3

Working with:
Unity 2018.1.0b2 , 2018.1.0b1 , 2017.3.0f3

Note:
1. Continous null reference exceptions are thrown in case of animation window, however, in case of animator window the error appears only once.
2. Animation window becomes unresponsive and user can't perform any operation on this window once these errors occur.
3. Clicking on a dropdown icon of Animation Window throws NullReferenceException and drop-down doesn't open.
4. On MacOs this does'nt happen always. Frequency is 1 out of 5.