You declare _args but never put anything to it so it's null.
Your code looks like it should be in a class that is called from a menu item button. In those cases you can use the Args class to pass values from your form to your class. Basically your menu item button will automatically send an Args object that contains the caller object (your form) and record info (selected record(s) from the data source that is referenced in your button group.
But if you write code directly on your form (like you do), no args are needed. You can just use the name of your form data source and call it directly.
So, if your form has a data source called InventTransferTable, the only line of code that you need is "info(inventTransferTable.TransferId);"
And you can also call InventTransferTable_ds.getNext() or any other method of the data source.