Salesforce: Fix broken Apex Trigger after an update to Accounting Seed

We have a salesforce system for about 30 employees and are using the Accounting Seed suite. At some point in the company's history we had some developers work on some custom apex triggers which assisted with automating payments. This has always worked fine but a recent update to our account suite "Accounting Seed" has caused the payment function to no longer work. The error was initially 'UpdateBSBAccount: execution of BeforeInsert caused by: [login to view URL]: Attempt to de-reference a null object ()' UpdateBSBAccount being the custom Apex trigger. To begin debugging I disabled the UpdateBSBAccount trigger to try and find out how the system worked. This allows the system to get to the next step but unsurprisingly the purpose that UpdateBSB was built for (which I believe, though I'm not sure, was to add BSB information and more info to the payment form) no longer works. I attempted to add it back now that I understood how it worked but I am unable to as it has 0 percent test coverage. Therefore the job is this. 1. Understand the debug logs in sandbox for why the updatebsb function shows that error 2. Resolve that error 3. Modify the existing tests (or write a new one) to give minimal code coverage for the trigger 4. Deploy the trigger with me to live. As it is simply an update bug I am assuming it is simple so this fee is for the fix, or if it turns out to be more complex it is for the rundown of what (vaguely) would be required to fix it, what the problem is and how much it would cost to resolve it.