C++ Acces violation reading error

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C++

Hey, I'm writing a C++ project, that is performing some graphical trasnformations.
I'm using a GPA.h class to create and manage pixel-arrays and Grid.h to break a bigger picture into a grid of smaller GPA-class pictures.

The function Grid::forge binds a grid of multipple GPA arrays into a bigger one.

GPA Forge()
	{
	GPA Result=GPA(xtotal,ytotal);
	for (int n=0;n<=X;n++){
	for (int m=0;m<=Y;m++){
	Result.Insert((*this)[n][m],0,0,n*xrange,m*yrange,xrange,yrange);
	}
	}
	return Result;
	}

It addresses the function, GPA::Insert

void Insert (GPA Source,int xSource, int ySource,int xHere, int yHere, int xArea,int yArea)
{
for(int m=0;m<yArea;m++){
for(int n=0;n<xArea;n++){
(*this)[xHere+n][yHere+m]=Source[xSource+n][ySource+m];
}
}
}

However when I execute the program it breaks with a error on the following line

(*this)[xHere+n][yHere+m]=Source[xSource+n][ySource+m];

on first execution. I also get a Unhandled exception: Access violation reading location. The members of (*this)[n][m] variable appear to be not initialized, even if the constructer was called and the varaible has been addressed before

Anyone got an idea why it happens?
Thanks in advance