It was possible that fetchNextTasks() would be called by multiple
threads concurrently.
If it happens, some tasks in the task queues might be ignored.
Change-Id: Idc81c43c45e382da3850cc55b9a42c281548d2a8
This fixes null pointer exceptions that are caused when
getSuggestion() is called from spell checker.
Change-Id: Ifc9e7abb900e1a4646e33c91022effde439673e4