What does the error "Expected the root reducer to be a function, but received 'undefined'" mean in Vanilla JS - Redux?

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This error typically occurs when the root reducer passed to the createStore function in Redux is undefined. This could be due to a number of reasons, such as a typo in the import statement for the root reducer file or a missing export for the root reducer function. It is important to ensure that the root reducer is properly imported and exported and that its function signature matches the requirements of the createStore function.