bug 441078: RAP: File Upload throws Widget is Disposed Exception

https://bugs.eclipse.org/bugs/show_bug.cgi?id=441078

Correctly dispose widget. (cherry picked from commit 6abcee25ab46985a7db7dd371e84617f7503d3ee)

Signed-off-by: Matthias Nick <Matthias.Nick@bsiag.com>