- Add null check to findEnumConstructorTarget

Change-Id: I1c2514d6c4602374a79adcdaad54a248ad4763a9
Signed-off-by: Kenneth Styrberg <kenneth@kean.nu>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.ui/+/180138
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Reviewed-by: Jeff Johnston <jjohnstn@redhat.com>