How can the error of "reducer" being a mandatory argument, which should be a function or an object of functions that can be utilized in combineReducers, be fixed?

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To fix the error of "reducer" being a mandatory argument, which should be a function or an object of functions that can be utilized in combineReducers, the following steps can be taken:

1. Check if the reducer argument is a function or an object of functions.
2. Make sure that the reducer argument is not null or undefined.
3. Ensure that the reducer argument has the correct syntax.
4. Check if any other arguments are missing or invalid.
5. If everything seems correct, try restarting the development server.