PropertyGrid throws NullReferenceException #613

Describe the bug
When i click on sorting buttons in PropertyGrid without selected object it throws NullReferenceException. Please add null-safety.

To Reproduce
Steps to reproduce the behavior:

1. Add PropertyGrid control
2. Set SelectedObject to null
3. Click on some sorting button

Expected behavior
When selected object is null, sorting buttons should just do nothing, not throw exception.

UPD: Also searchbar is going to throw exception if you will try to type something