Since this error is only occurring when you're trying to open a particular bill, it means that this bill has become corrupted.
The only way around this is to delete the bill from the register in the chart of accounts and enter it again (by doing it like this, you do not need to open the bill). See below instructions.
1. Go to your chart of accounts. 2. Right-click on the account that is associated with this bill > click use register 3. Find this particular bill and take note of the details so you can enter it in again 4. Click on the bill once in the register (it should be highlighted with a black box around the transaction) 5. On your keyboard hold down ctrl + D together > it will prompt you if you are sure you want to delete this transaction > click yes 6. Enter the bill again