Don't crash when we fail to find a screen for the active tooltip

There's a hidden hidpi issue lurking in this code base. What's odd
is that it's not easily reproducible everywhere. For now, prevent
a crash when we fail to find a screen at the tooltip's location.

CCBUG: 417151
(cherry picked from commit e37549a4b04cd88174b968b37c7061376e0c6744)