2.80, a3261aad130e, 2019-01-04
2.79, f4dc9f9d68b, 2018-03-22
Short description of error
There is a null pointer exception in the search callback for properties. It happens when the mouse cursor is not moved off the property after a drop event and the selection menu is opened instead.
Exact steps for others to reproduce the error
Open default startup .blend.
Duplicate the camera.
Select Scene tab in Properties window.
Select "Camera.001" as Camera.
Drag "Camera" from Outliner window to the Camera property in Scene tab.
Don't move the mouse. (Moving the mouse away from the button avoids the exception)
Click to open the selection menu.
Setting but->free_search_arg to false during drop will fix the null pointer exception but probably introduces memory that is never freed.