PIVOT-938:  Further checks in the vein of PIVOT-927, where sometimes
the mouse location can be null.  We still haven't quite figured out the
underlying cause, but guarding against a null pointer access seems to
work around the issue without causing too much harm.

This latest incarnation has to with displaying tooltips, where the
mouse location can also get to be null.


git-svn-id: https://svn.apache.org/repos/asf/pivot/branches/2.0.x@1580958 13f79535-47bb-0310-9956-ffa450edef68