tmf.ui: Gracefully handle invalid tar import operations

tmf.ui: Gracefully handle invalid tar import operations

A tar.gz file may have an invalid tar file in the gzip. The
getNextEntry would return null in such a case. This performs
that null check in order to validate the tar and avoid a user-
facing NPE.

Change-Id: I28ae7c787d77a0f1c9136264cfc678d7e52cda1a
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/87247
Reviewed-by: Hudson CI
Reviewed-by: Marc-André Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-André Laperle <marc-andre.laperle@ericsson.com>