Tasks with precedence restriction to a task that does not exist should be dropped

[Markusdreyer]

If a task (T2) has a precedence restriction to task (T1), and T1 does not exist, then T2 should be dropped. This implicitly means that any tasks that has a precedence restriction to T2 should also be dropped.

Current behaviour is that the task is included, regardless if the tasks precedence restriction is present in the result set. See example below showing task T2 in the result set, even though T2 has a precedence restriction to T1, which does not exist.