Object reference not set to instance of an object .... while running stsadm.exe in adding the solution through wsp etc

The problem here is that the user you are logged in with does not have the required permissions for the database server.
Note: Remote Login -> Sql Server Management Studio --> Connect to Database --> Logins --> New Login --> Mappings, Server Roles etc.
Note: In User Mappings, Give db_owner role for all the databases needed.
If you run this with the same account as you installed your web applications with, there should be no problem.
You will need full control to the content database.
stsadm always uses credentials of the logged in user, unless offcourse you open your command prompt like this “runas /user:mydomain\myuser cmd.exe”.
Exchange mydomain and myuser with the values for the user you want to open a command prompt as.