Always use breakpoints for troubleshooting especially when you know at what line of code the program stops with an error.
Hover the mouse over the variables at the line and on the lines above, to see what values they have.
You can put several breakpoints and follow the flow of the execution by pressing F11.
The attached picture shows an example of using these, where red dots are the breakpoints.
The yellow highlighted line is where the program execution is currently.
The lines below the highlighted line have not been processed yet but the variables in these lines could still show some value.
Use Google to find out more about the breakpoints.
