California income tax withheld from remote (non-resident) job

This past year, I worked for a company based in California. I was a remote employee and did all my work from my home state of Minnesota. It is my understanding that the income source for this job is MN and not CA, and as such is subject to MN income tax but not CA income tax.

The complication I am facing is that instead of withholding MN income tax, the company withheld CA income tax from my paychecks. (I asked about this when I first started, but I was told that CA withholding was correct, and I didn't know enough at that point to question it further.) So for this year's taxes, I need to file a CA state return to get a refund on that withholding, and then pay MN income tax on that income.

My questions:

1. Is fixing this as simple as filling out form 540 NR and putting $0 for my California income? 2. Do I need to provide any evidence or supporting documentation that the income in question had a source in MN and not CA? 3. Up until this year I have filed my taxes on my own, and am comfortable doing so again this year - does using a professional tax preparer gain me anything in this situation?