You may already be aware that budget snapshots are required for the use of Earned Value, but you should also know that budget snapshots may be required for revenue recognition if you have Fixed Price items where you are recognizing revenue other than when billed. This would be if you are recognizing revenue based on % Complete or some custom revenue recognition schedule. At a minimum, you need to create a single budget snapshot that represents the previous % Complete value.
Note that the initial snapshot should be a 0 and not null value:
Navigate: Projects > List > List tab > Edit pencil icon > Budget Snapshots > Manage.