main.js:1 ERROR Error: StaticInjectorError(r)[n -> n]: code example

Example: ERROR Error: Uncaught (in promise): NullInjectorError: StaticInjectorError(r)[n -> n]: StaticInjectorError(Platform: core)[n -> n]: NullInjectorError: No provider for n!

ERROR Error: Uncaught (in promise): NullInjectorError: StaticInjectorError(r)[n -> n]: 
  StaticInjectorError(Platform: core)[n -> n]: 
    NullInjectorError: No provider for n!