QMLDebugging: Fix crash

Check for null pointers when parsing AST to relocate breakpoint

Change-Id: I484048d9e8a7aabdeffc4e18049c9b9b6f030366
Reviewed-by: Simjees Abraham <simjees.abraham@nokia.com>
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>