Fix null reference in AddCommandHandlerCallable when callable does not return an object

[meybax]

When registering a command without a return value or that returns primitives, handling the command fails with a null reference exception when awaiting the asynchronous task. This fix only checks for an asynchronous callable if the return type is a GodotObject.

[dogboydog]

This makes sense to me. Thanks for the fix. I will want to make sure to port this to development-v3 as well

[dogboydog]

Actually I think it might already be handled in development-v3 , because all the references to GDScriptFunctionState are like this

     if (methodReturn.Obj != null && methodReturn.As<GodotObject>().GetClass() == "GDScriptFunctionState")
            {

Which checks for the null in a different way. But I'll try to remember to make sure to test again that this works in 0.3.0 when the methods return nothing