Yes, in general grid tie PV inverters should only be used connected to the grid, and not to a generator.
As posted above, the grid tied PV inverter should be connected to the line side of the transfer switch, such that it is connected to the grid but not to the generator.
In most cases, the generator frequency and voltage will not be accurate enough for the inverter to function.
If however the generator output WAS accurate enough for the inverter to run, then serious harm could result at no or low load.
If the PV inverter was producing say 20KW and the load was 60KW, then the generator would supply the other 40KW and all would be well.
If however the load reduced to only 5KW, then the inverter would "try" to feed 15KW back into the generator. Serious harm might then result since unlike the grid, the generator is not designed to absorb power, only to supply it.
I have used a grid tied inverter with a generator, and it worked fine.
It was however a 650KVA generator and a 4KVA inverter, and I was satisfied that the load would never be less than 4KVA.
This was however due to a mistake by others, and I can not recomend the practice.