Don't know exactly what caused the bug/issue but I submitted a support ticket referencing this thread. Last week they replied with the problem will be fixed in the next 4.2.x release. I haven't heard yet when that is scheduled.
Hello, I've actually found the same behaviour but only if the grid store has never had results. Once you populate the store with something which returns at least one result then you can change the params and load the store with a variables that will return 0 results and this error doesn't seem to occur.