ExportAllApplicationAddin.cs: fix crash when a linked note is not found
ExportAllApplicationAddin.cs: fix crash when a linked note is not found

Fixes #25.

Signed-off-by: Alex Tereschenko <10ab4a620366487513eb6142e6429449f497066c@gmail.com>

tomboy-notes/tomboy,tomboy-notes/tomboy,tomboy-notes/tomboy,tomboy-notes/tomboy,tomboy-notes/tomboy