Trying to read undefined view.mainview in mainview.js

I'm not sure if that is intended and the problem was on my side, but I had to fix this to get the admin starting.

If the `view.mainview` variable is not defined, `!view.mainview` evaluates to true so the boolean expression continues to test with `!view.mainview.show`, which of course throws an error because node can't call a value from undefined.

You can easiely rewrite that as `(view.mainview && view.mainview.show)`, because in this case `view.mainview` evaluates to false if it's undefined, and so the second part is not even checked (because it's an AND).

But as it appears also the `view` variable can be undefined, so I had to use `(view && view.mainview && view.mainview.show)`.

That way this boolean expression represents the exact opposite of the original one which made it easy to also just omit the `continue;` part.


Here the original output with **errorstack**. The error appears after logging in, thats why the `/login` route logged normally.
```
GET /login 200 11ms - 7.68kb
POST /login 302 268ms - 58b
TypeError: Cannot read property 'mainview' of undefined
    at exports.get (c:\Users\Phil\Documents\GitHub\express-admin\routes\mainview.js:15:18)
    at callbacks (c:\Users\Phil\Documents\GitHub\express-admin\node_modules\express\lib\router\index.js:164:37)
    at exports.restrict (c:\Users\Phil\Documents\GitHub\express-admin\routes\auth.js:20:34)
    at callbacks (c:\Users\Phil\Documents\GitHub\express-admin\node_modules\express\lib\router\index.js:164:37)
    at param (c:\Users\Phil\Documents\GitHub\express-admin\node_modules\express\lib\router\index.js:138:11)
    at pass (c:\Users\Phil\Documents\GitHub\express-admin\node_modules\express\lib\router\index.js:145:5)
    at Router._dispatch (c:\Users\Phil\Documents\GitHub\express-admin\node_modules\express\lib\router\index.js:173:5)
    at Object.router (c:\Users\Phil\Documents\GitHub\express-admin\node_modules\express\lib\router\index.js:33:10)
    at next (c:\Users\Phil\Documents\GitHub\express-admin\node_modules\express\node_modules\connect\lib\proto.js:193:15)

    at c:\Users\Phil\Documents\GitHub\express-admin\node_modules\express\lib\application.js:121:9
GET / 500 14ms - 1.1kb
```