NullReferenceException

[hloQt]

""An unhandled exception of type 'System.NullRef erenceException ' occurred in WindowsFormsApp lication2.exe

Additional information: Object reference not set to an instance of an object."

I am new at programming and I don't really get the explanation that I saw on the sites.
When I am going to use a Button.performc lick operation, I do get this problem. I clearly do not see any problems in my code but please take a look in it.

 {
            
                Button z = null;
                
                z = find_win_or_block("O");

                if (z == null){
                
                    find_win_or_block("X"); if (z == null){
                    
                        find_corner();
                        if (z == null){
                        
                            find_open();

                        }
                    }
                }
               z.PerformClick();
            
            
          

       }
        public Button find_win_or_block(string mark)
        {
            Console.WriteLine("Find win or block " + mark);
            if (a1.Text == mark && a2.Text == mark && a3.Text == "")
                return a3;
            if (a2.Text == mark && a3.Text == mark && a1.Text == "")
                return a1;
            if (a3.Text == mark && a1.Text == mark && a2.Text == "")
                return a2;
           
            if (b1.Text == mark && b2.Text == mark && b3.Text == "")
                return b3;
            if (b1.Text == mark && b3.Text == mark && b2.Text == "")
                return b2;
            if (b2.Text == mark && b3.Text == mark && b3.Text == "")
                return b1;

            if (c1.Text == mark && c2.Text == mark && c3.Text == "")
                return c3;
            if (c2.Text == mark && c3.Text == mark && c1.Text == "")
                return c1;
            if (c2.Text == mark && c1.Text == mark && c3.Text == "")
                return c3;
            //8=======D
            if (a1.Text == mark && b1.Text == mark && c1.Text == "")
                return c1;
            if (a1.Text == mark && c1.Text == mark && b1.Text == "")
                return b1;
            if (b1.Text == mark && c1.Text == mark && a1.Text == "")
                return a1;

            if (b2.Text == mark && a2.Text == mark && c2.Text == "")
                return c2;
            if (c3.Text == mark && b2.Text == mark && a2.Text == "")
                return a2;
            if (a2.Text == mark && c2.Text == mark && b2.Text == "")
                return b2;

            if (c3.Text == mark && b3.Text == mark && a3.Text == "")
                return a3;
            if (a3.Text == mark && c3.Text == mark && b3.Text == "")
                return b3;
            if (a3.Text == mark && b3.Text == mark && c3.Text == "")
                return c3;
            //OIOIOIOIOIOIOIOIXXXIOIOIOIOIOIOIOIOIO
            if (a1.Text == mark && b2.Text == mark && c3.Text == "")
                return c3;
            if (a1.Text == mark && c3.Text == mark && b2.Text == "")
                return b2;
            if (b2.Text == mark && c3.Text == mark && a1.Text == "")
                return a1;

            if (a3.Text == mark && b2.Text == mark && c1.Text == "")
                return c1;
            if (b2.Text == mark && c1.Text == mark && a3.Text == "")
                return a3;
            if (c1.Text == mark && a3.Text == mark && b2.Text == "")
                return a3;

            return null;
            
        }
        public Button find_corner()
        {
            if (a1.Text == "O")
            {
                if (a3.Text == "")
                    return a3;
                if (c3.Text == "")
                    return c3;
                if (c1.Text == "")
                    return c1;
            }
            if (a3.Text == "O")
            {
                if (a1.Text == "")
                    return a1;
                if (c3.Text == "")
                    return c3;
                if (c1.Text == "")
                    return c1;
            }
            if (c1.Text == "O")
            {
                if (a3.Text == "")
                    return a3;
                if (c3.Text == "")
                    return c3;
                if (a1.Text == "")
                    return a1;
            }
            if (c3.Text == "O")
            {
                if (a3.Text == "")
                    return a3;
                if (a1.Text == "")
                    return a1;
                if (c1.Text == "")
                    return c1;
            }
            if (a3.Text == "")
                return a3;
            if (c3.Text == "")
                return c3;
            if (c1.Text == "")
                return c1;
            if (a1.Text == "")
                return a1;
            
            return null;
        }
        public Button find_open()
        {Console.WriteLine("Find open ");
            Button qwerty = null;
            foreach(Control a in Controls)
            {
                qwerty = a as Button;
                  if(qwerty!=null)
                {
                    if(qwerty.Text=="")
                    return qwerty;
                }
            }
            return null;
            }

[zmbd]

Your posted code has close to 150 lines, expecting someone to just step thru it is a bit much without some further details:

>> What have you already done to troubleshoot this script?

>> Have you stepped thru the script using the debugger?
MS Visual Studio: Navigating through Code with the Debugger - at what point do you receive the error?

>> How are you creating your project? Visual Studio, Visual Studio Express or some other ISE and what version?)

This information will help our experts to pinpoint the problem.