When a declaration and an implementation disagree on nullability, a warning is produced.
Given It would be convenient to have a one-click solution to either:

• change the declaration to match this implementation (this would be especially convenient because navigating to the declaration isn't easy at the moment),
• change this implementation to match the declaration.

There is a similar situation with overrides, but at least there, you can easily GoToDefinition to find the declaration.

Note: this fixer should not have a FixAll.