"Object reference not set to an instance of an object" error while opening nworks UI

It is necessary to remove the "maindb.xml" file in the following way: 

1. Locate and backup file "C:\Documents and Settings\All Users\Application Data\nworks\Management Center\maindb.xml" (for Win 2008 the location is "C:\ProgramData\nworks\Management Center”)

2. Stop nworks Enterprise Manager (Management Center) service

3. Delete file "C:\Documents and Settings\All Users\Application Data\nworks\Management Center\maindb.xml" (for Win 2008 the location is "C:\ProgramData\nworks\Management Center”)

4. Start nworks Enterprise Manager (Management Center) service 

Once the service is started again, "maindb.xml" will be recreated. 

NOTE: Removing maindb.xml file removes all the vCenter connection configuration, each VEM server advanced property customization, and VEM server ESX cluster assignments.