Some more code around the exception line will help. But there are three possibilities right now:
1: You've created a form dynamically without the 'New' keyword, which in result is throwing a null reference error. Use the 'New' keyword before creating a new dynamic form.
2: If you've created the form with out dynamic code, then unload the Form from the project. Close the project and then reload it. Now add the Form you previously uploaded, and then call the "FormName.show()" Method. This will hopefully update the project and will recognize it.
3: If you've some code on the load event of the form you are showing then 90 % chance is that, that code is throwing an exception. To solve this, modify the code or wrap it around the Try...Catch block.
Take into account these possibilities and try to solve them. If the error still persists, then we gotta need some code of the project.
Hope this helps !!