After creating several tasks and try to add some to related tasks the drop down list appear empty until saved task are opened for edit again.

Steps:
1 Create and save task 1
2 create and save task 2
3 create and save task 3
4 go task 1
5 click on add related task
6 select local task
7 Drop down list appear empty
8 close and open task 2
9 close and open task 1
9 click on add related task
10 select local task
11 only task 2 appear
12 close and open task 3
13 repeat step 11, task 3 appear