[LyX/2.2.x] Fix typo, and also check, if string is null.

[Richard Heck]

commit bfcd8a3991a6b4e175d5f368ef7ed1de6fa17c00
Author: Richard Heck <rgh...@lyx.org>
Date:   Sat Jun 18 18:34:30 2016 -0400

    Fix typo, and also check, if string is null.

diff --git a/src/frontends/qt4/GuiRef.cpp b/src/frontends/qt4/GuiRef.cpp
index 795736c..549e3cc 100644
--- a/src/frontends/qt4/GuiRef.cpp
+++ b/src/frontends/qt4/GuiRef.cpp
@@ -118,7 +118,8 @@ void GuiRef::gotoClicked()
        // to which we are going (or from which we are returning) is
        // restored in the dialog. It's a bit of a hack, but it works,
        // and no-one seems to have any better idea.
-       bool const toggled = last_reference_.empty();
+       bool const toggled = 
+               last_reference_.isEmpty() || last_reference_.isNull();
        if (toggled)
                last_reference_ = referenceED->text();
        gotoRef();