How to fix "object reference does not point to an instance of an object" error?

Question

C++ / C#

Windows Forms. Program "Calculator". When clicking on a TextBox or any empty area on the form (except buttons), debugging is interrupted by the message "Unhandled exception of type 'System.NullReferenceException'. Additional information: 'Object reference does not point to an instance of an object.' why the program reacts this way, especially it is not clear why this happens when you click on an empty (!) area of ​​the form where no components are placed.I wrote event handlers only for Buttons.

Answer 1

This error means that instead of an object - null.

1. Run the program in debug mode;
   
2. Reproduce the error;
   
3. Follow the link to the specified point in the source code;
   
4. You fix the error.