Undefined reference to function from static pointer

I'm experimenting around with dll loading on runtime and I have a problem: I have a little helper class which is istanciated in the main program. The pointer to this object is handed to the loaded dll. To test it, i want to call a function from the class (printLine). But I can't compile the dll because I get a:

Utility.o: In function `ZN7Utility6onInitEv':
D:\Benutzer\Jan\Desktop\Programmierprojekte\Game Engine 5.0\Utilities\Debug/../Utility.cpp:7: undefined reference to `ModuleHelper::printLine(std::string)'
collect2.exe: error: ld returned 1 exit status

These two files are the only content of the dll:

Utility.h:

#ifndef UTILITY_H_
#define UTILITY_H_

#include <iostream>
#include <Definitions.h>
#include <ModuleHelper.h>

class Utility
{
public:
    Utility();
    ~Utility();

    static void onInit();
    static void onUpdate();

    static char* getName();
    static char** getDependencies();
    static int getCountDependencies();
    static char* getServeAs();

    static void setModuleHelper(ModuleHelper* helper);

private:
    static constexpr char* name = "Utility";
    static constexpr char** dependencies = nullptr;
    static constexpr int countDependencies = 0;
    static constexpr char* serveAs = "";
    static ModuleHelper* moduleHelper;
};

extern "C" //GAME_API is a dllexport macro
{
    char* GAME_API getName()
    {
        return Utility::getName();
    }

    char** GAME_API getDependencies()
    {
        return Utility::getDependencies();
    }

    int GAME_API getCountDependencies()
    {
        return Utility::getCountDependencies();
    }

    char* GAME_API getServeAs()
    {
        return Utility::getServeAs();
    }

    noargfunc GAME_API onInit()
    {
        return Utility::onInit;
    }

    noargfunc GAME_API onUpdate()
    {
        return Utility::onUpdate;
    }

    void GAME_API setModuleHelper(ModuleHelper* moduleHelper)
    {
        Utility::setModuleHelper(moduleHelper);
    }
}
#endif /* UTILITY_H_ */

Utility.cpp:

#include "Utility.h"

ModuleHelper* Utility::moduleHelper; //with "= nullptr" or "= NULL" it didn't work either

void Utility::onInit()
{
    moduleHelper->printLine("Hello from Utilities"); //wrapper for std::cout
}

void Utility::onUpdate()
{

}

char* Utility::getName()
{
    return name;
}

char** Utility::getDependencies()
{
    return dependencies;
}

int Utility::getCountDependencies()
{
    return countDependencies;
}

char* Utility::getServeAs()
{
    return serveAs;
}

void Utility::setModuleHelper(ModuleHelper* helper)
{
    moduleHelper = helper;
}