Top-block does not find Probe Signal block

Hello GNUR’s!

I am currently working with a SDR dongle which I want to lock at four
different frequencies to see four consecutive parts of the spectrum. I
have to do so because the sampling rate is not enough (2.4 MHz) to see
at one glance the bandwidth I need (8 MHz). To achieve so I am
multiplexing the values I want to lock my dongle to but I am getting
some errors using the pair of blocks “Probe Signal” and “Function Probe”
(see the flowgraph in [1] and errors trace [2]). By enabling/disabling
some blocks I have guessed that the error about the overflow is caused
by the WX GUI Number Sink, while the error of the non-existing attribute
appears when I make use of the “variable” defined by the block Function

Of course I have followed the tutorial [3] and still couldn’t make it to
Thank you very much in advance.