Fix for dBConverter throwing"InvalidCastException"

[Federerer]

On current master, when you try manually input a value for "Input gain" or "Output gain" during the simulation an unhanled exception is thrown: This PR fixes it.

[dsharlet]

I see the problem on master, but when I try the fix, it just highlights the text box in red, even for valid inputs. The sliders also don't seem to work either. I've tried a few fixes but no luck so far. I'll try to poke at it for a bit more too.