I figured it out.
In case someone else gets stuck at this point. When calling synchronously, the parameters of the function must be complete:

const var th = Engine.createTransportHandler(); th.setSyncMode(1); th.setEnableGrid(true, 5); inline function onGrid(grid, timestamp, first) // Parameters need to be complete { Console.print(grid); } th.setOnGridChange(true, onGrid);