namespace "core::systemstate" has no member "Manager"

Question

When I include app.hpp in manager.hpp it throws error as namespace has no member error in app.hpp after removing #include it vanishes. But I need to create pointer of app class in manager class and vice-versa.

Note : Both below classes are in different namespace

//App.hpp

#include "Manager.hpp"

namespace core{
namespace  application{

class App {
public:
    App();
    virtual ~App() = 0;
    
    //defined in derived class
    virtual void onStateChange(u32 fromState, u32 toState) = 0;
protected:
    core::systemstate::Manager *State;
        };
    }
}

//Manager.hpp

#include "App.hpp"
namespace core
{
    namespace systemstate
    {
        class Manager
        {
                public:

                Manager();
                
                ~Manager();
                
                void initializeManager();
        }
    }
}

D:code\app\include\App.hpp", error #135:

namespace "core::systemstate" has no member "Manager"

core::systemstate::=Manager *State;

Note : Both below classes are in different namespace