Our crash would occur when changing the filter of a txn or entry view
that resulted in an smaller list than what we had before. The `_fill()`
method, which would happen after our `core` part of the list was
filtered but before our `ui` part of the list was refreshed, would
trigger a UI update through scrolling. Then, this UI update would
trigger an information fetch for an index that didn't exist, our core
table having less rows than our ui table.
This bug could only be reproduced on newer versions of qt (Ubuntu
16.04), which is surprising because the behavior being changed in this
commit was clearly faulty.