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.

This is a merge of revision 1580958 from branches/2.0.x to trunk.



git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1583846 13f79535-47bb-0310-9956-ffa450edef68