fix(Interaction): ensure highlighter exists before highlighting
fix(Interaction): ensure highlighter exists before highlighting

The highlighter gets destroyed in `OnDisable` and on a rare occasion
there can be a race condition where the highlighter tries to highlight
even though the highliter has been destroyed.

This fix ensures the objectHighlighter exists before attempting the
highlight.

thestonefox/VRTK,thestonefox/SteamVR_Unity_Toolkit,TMaloteaux/SteamVR_Unity_Toolkit,pargee/SteamVR_Unity_Toolkit,adamjweaver/VRTK,virror/VRTK,gpvigano/VRTK,gpvigano/SteamVR_Unity_Toolkit,Innoactive/IA-unity-VR-toolkit-VRTK,nmccarty/htvr,red-horizon/VRTK,Fulby/VRTK,mattboy64/SteamVR_Unity_Toolkit,gomez-addams/SteamVR_Unity_Toolkit