NPrinting Designer Error Object reference not set to an instance
When you have two or more NPrinting connections using the same source QVW or QVF and BOTH connections are contained within the SAME NPrinting 'App', the following error is generated when attempting to preview an NPrinting report in the NPrinting Designer:
Error Object reference not set to an instance
NPrinting November 2020 and earlier versions
This was not supported option in earlier versions of NPrinting as per the stated connection rules for both QlikView and Qlik Sense. Doing so in earlier versions results in the error shown above.
"You can only create one connection between a Qlik Sense app and a Qlik NPrinting app."
Support for multiple connections using the same QVF or QVW inside a single NP App began in Feb. 2021 version of NPrinting. You can view the fix/new feature in the Feb. 2021 release notes - attached. See OP-9606
To prevent this error, upgrade your NPrinting Server and Designers to February 2021 NPrinting or later releases