dApp: getting channel with biggest capacity is insecure if no tokens exist

[weilbith]

This is a more of a theoretic issue, but super simple to fix.

Steps to Reproduce

1. Attempt to get the channel with the biggest capacity for a token that is not used
2. Checkout the error message of the store its getter function

Expected Result

• Return something like null (like in other getters)

Actual Result

• Throws