Why does this happen?
This error occurs on the EditInvoiceItem screen when you're adding a new line item to your invoice. This will occur when you hit "Save" before the device is able to load all of the Pricebook details for that item.
How can I fix this?
This can be avoided simply by waiting an extra few seconds before hitting save. The load time of the invoice item varies greatly on the speed of your device. If you are seeing that it takes a while to load, please make sure all unnecessary applications are closed in the background of your device.