I initially had this issue at the start of the game, when I was on the ship. If I walk away and come back much later, it's stopped drawing on the screen. Now I'm getting it in combat, particularly on the Defiant, but also in some encounters such as back alley ones.
I'm seeing spam of this (many many times) in the output log:
(Filename: Line: -1)
NullReferenceException: Object reference not set to an instance of an object
at Game.UI.UIImageButtonRevised.get_MousedColor () [0x00000] in <filename unknown>:0
at Game.UI.UIImageButtonRevised.NotifyModeChanged () [0x00000] in