Comment 74 for bug 372103

Well, after some investigation I believe the NULL check is a proper solution here.

For instance, see the clipboard tutorial at http://www.gtk.org/tutorial1.2/gtk_tut-19.html

Anyway, selection_data and its description by atoms is created by application which pastes data into clipboard. Mozilla has not any mechanism how to deny third-party app to paste incorrect targets into clipboard. We can only ignore them.