Touchpad debug view: fix crash when disabling using touchpad

When using the touchpad to toggle the "Show touchpad input" developer
option off, the hideDebugView method (which sets mTouchpadDebugView to
null) was running in between the mTouchpadDebugView.post call and the
posted action actually running, so we need another null check within the
action.

Bug: 376018148
Test: with the developer option enabled, click its toggle using the
      touchpad that's currently being visualized
Flag: com.android.hardware.input.touchpad_visualizer
Change-Id: I8e2be39c05c9cab2d51b98acabf95b399d0a00a5