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Debug assertion failed! Invalid null pointer.

Hagge96 (1)
I got a school assignment where I need to encrypt what the user writes in and then decrypt it againt. I can run this code so it starts but when I come to the point where I write in the text that should be encrypted I get a window pooping up saying:
"Debug Assertion Failed!

Program:...cts\Programmering Projekt C\Debug\Programmering Projekt C.exe
File:c:\program files (x86)\microsoft visual studio 10.0\vc\include\xstring
Line:929

Expression: invalid null pointer

For information on how your program can cause an assertion failure, see the Visual C++ documentation on asserts.

(Press Retry to debugg the application)"

I can't find anything on how to fix this or what is creating this error, I would really appreciate some help with this. 

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  #include <iostream>
#include <string>	
#include <iomanip>	
#include <istream>
using namespace std;

int val, storlek, rot, start, längd;
char x;
double samtalspris(), momsfunktion();
double telefonsumma, Totaltelefonsumma, sluttid, helminuter2;												
double rabattsumma, telefonsumma1, pris, helminuter1, starttid,  totalminuter;
string delsträng, timmar1, minuter1, timmar2, minuter2, kryptering(), dekryptering(), text;													
double rabatt = 0.15;																		
double kostnad = 3;																				
double moms = 1.25;	

int main()
{
switch (val)
{	
	case 1:	
		cout<<"Skrev in texten du vill ha krypterad: ";
			getline(cin, text);
		cout<<endl;
		cout<<"Din text krypterad är: ";
		cout<<kryptering();
		cout<<endl;
			text=kryptering();
		cout<<"Din text dekrypterad är: ";
		cout<<dekryptering();
		cout<<endl;
		main();
		break;
}
string kryptering()																				
{
	for( int i=0; i<text.size(); i++)
	{
		if(i%5==0)
		{
			 x*=-1;
		}
		if (x==-1)
		{
			if (text[i]>='a'&&text[i]<='m')
			{
				text[i]-=7;
			}
			else if (text[i]>='m'&&text[i]<'z')
			{
				text[i]+=7;
			}
			else if (text[i]>='A'&&text[i]<='M')
			{
				text[i]-=7;
			}
			else if (text[i]>='M'&&text[i]<='Z')
			{
				text[i]+=7;
			}
		}
		else (x==1);
		{
			if (text[i]>='a'&&text[i]<='m')
			{
				text[i]-=13;
			}
			else if (text[i]>='m'&&text[i]<='z')
			{
				text[i]+=13;
			}
			else if(text[i]>='A'&&text[i]<='m')
			{
				text[i]-=13;
			}
			else if(text[i]>='M'&&text[i]<='m')
			{
				text[i]+=13;
			}

		}

	}
	return 0;																				
}																								

string dekryptering()
{

	for( int i=0; i<text.size(); i++)
	{
		if(i%5==0)
		{
			 x*=-1;
		}
		if (x==-1)
		{
			if (text[i]>='a'&&text[i]<='m')
			{
				text[i]+=7;
			}
			else if (text[i]>='m'&&text[i]<'z')
			{
				text[i]-=7;
			}
			else if (text[i]>='A'&&text[i]<='M')
			{
				text[i]+=7;
			}
			else if (text[i]>='M'&&text[i]<='Z')
			{
				text[i]-=7;
			}
		}
		else (x==1);
		{
			if (text[i]>='a'&&text[i]<='m')
			{
				text[i]+=13;
			}
			else if (text[i]>='m'&&text[i]<='z')
			{
				text[i]-=13;
			}
			else if(text[i]>='A'&&text[i]<='m')
			{
				text[i]+=13;
			}
			else if(text[i]>='M'&&text[i]<='m')
			{
				text[i]-=13;
			}

		}

	}
	return 0;							
}
tipaye (535)

A lot is wrong here really, but one big problem (lines 84 and 137) is that you're returning a 0 instead of a string.
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