The problem is that the function in your server-side controller requires an account as input:

    public static Account saveAccount(Account acc)

You are setting this parameter on the action, but you are using the value of what is typed into the `ui:inputText` box, which even if it is an account id, it is not an account object. 

    var newAccount= component.find("AccountId").get("v.value");
    action.setParams({"acc" : newAccount});

---

Assuming you wanted to query the details of an account based on it's id, you would want to change the method in your server-side controller to accept an Id, perform a SOQL query, and then return the results. 


    public with sharing class AccControllerClass {
        @AuraEnabled
        public static Account saveAccount(Id acc){  // this name should be changed.
            if (acc == NULL){                       // I left it the same to avoid having to change the lightning component.
                AuraHandledException e = new AuraHandledException('Please input an id');
                e.setMessage('Please input an id');
                throw e;
            }
            Account a = [
                SELECT Id, Name
                FROM Account
                WHERE Id = :acc
            ];
            return a;
        }
    
    }