Flutter Navigator not working

Think of the widgets in Flutter as a tree, with the context pointing to whichever node is being built with the build function. In your case, you have

MainScreen    <------ context
  --> MaterialApp
   (--> Navigator built within MaterialApp)
      --> Scaffold
        --> App Bar
          --> ...
        --> Center
          --> FlatButton

So when you’re using the context to find the Navigator, you’re using a context for the MainScreen which isn’t under the navigator.

You can either make a new Stateless or Stateful Widget subclass to contain your Center + FlatButton, as the build function within those will point at that level instead, or you can use a Builder and define the builder callback (which has a context pointing at the Builder) to return the Center + FlatButton.