Placing frequency block segfaults

[helaslo]

Probably similar to #1011

Reproducer steps:

1. Open latest ngscopeclient
2. Import this CSV (had to upload via a repo compressed, GitHub didn't let me attach it as a comment attachment)
3. Create a frequency measurement block from it in the filter graph
4. Segfault

Additional note:

• When placing frequency measurement block, then deleting it, there is some logs about dangling references
• This CSV input file prevent running FFT on a different PC, will create an issue for it soon