Object Reference Not set to an Instance of an object

Hi ,

      I created an exe . It contains around 10 forms. When I click back button in a form it will go to form2. For example I wrote

the following code in form4 for going back to form2

           this.Hide();
            Form2 FF = new Form2();
            
            FF.ShowDialog(this.Owner);

But when I run the program on my system, it works fine. After that I installed the exe on another system . In that system when I press the back btton on form4, it will go to form2 but shows the error that "Object Reference Not set to an Instance of an object". How can I solve this problem?? Is any updation needed for my code??

Thank you.