Why must I run cvx_setup again after I modified Mosek as default solver in my code?

I seted up cvx successfully, then i can run my code which i modified mosek as default solver the fisrt time. But then when i run my another cvx code, it says
Saving prefs…done.
Undefined function or variable ‘variable’.

Error in myThird (line 13)
variable lambda(n,n);

If I run cvx_setup again, there will be no that error.
So how can i do solve the problem?