tmf: Bug 436738: NPE after deleting default custom parser

tmf: Bug 436738: NPE after deleting default custom parser

- Reload the default custom parser (if it exists) after deleting any
custom parser

- Add null checks when getting a trace type helper from a trace type id

Change-Id: If2b947ff6ce18371e9fdbe5a68f02a1b650fc27a
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/28067
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
(cherry picked from commit ed74d892ad2dfaafd64c43b6a1b619d402780438)
Reviewed-on: https://git.eclipse.org/r/28075