[SalesForce] Passing Parameter Value to the Apx(server-side controller) in lightning

By a "null error", I'm assuming you mean a null pointer exception. If that's the case, you'll want to use a try-catch block where you catch the null-pointer exception. I'd also recommend you change your If condition to isEmpty. Your code in the server side controller would look something like this:

try{

if(!isEmpty(caseRecord.Freight_AWB_Number__c){
            requestJson = getRequestString(username, password, caseRecord.Freight_AWB_Number__c);
        }
}catch(nullpointerexception e){

            // show error AWB Number not provided. Lightning work to be done
            msg = 'AWB Freight Number cannot be null';
            errH = new ErrorHandler(false,msg);
            return errH;
}

I'm assuming you are successfully calling the JS Controller's IdAccount() function from somewhere. If that is the case, then you are not successfully calling your server side method because you did not add the server side action to the queue.

Here's some sample code which shows you the correct way to call a method from your server side controller:

({
    "echo" : function(cmp) {
        // create a one-time use instance of the serverEcho action
        // in the server-side controller
        var action = cmp.get("c.serverEcho");
        action.setParams({ firstName : cmp.get("v.firstName") });

        // Create a callback that is executed after 
        // the server-side action returns
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (state === "SUCCESS") {
                // Alert the user with the value returned 
                // from the server
                alert("From server: " + response.getReturnValue());

                // You would typically fire a event here to trigger 
                // client-side notification that the server-side 
                // action is complete
            }
            else if (state === "INCOMPLETE") {
                // do something
            }
            else if (state === "ERROR") {
                var errors = response.getError();
                if (errors) {
                    if (errors[0] && errors[0].message) {
                        console.log("Error message: " + 
                                 errors[0].message);
                    }
                } else {
                    console.log("Unknown error");
                }
            }
        });

        // optionally set storable, abortable, background flag here

        // A client-side action could cause multiple events, 
        // which could trigger other events and 
        // other server-side action calls.
        // $A.enqueueAction adds the server-side action to the queue.
        $A.enqueueAction(action);
    }
})

Notice the 3 main steps:

1. Create instance of server side action: var action = cmp.get("c.serverEcho");
2. Create a callback that is executed after server side action returns: action.setCallback(this, function(response) {...
3. Add the server side action to the queue to be executed: $A.enqueueAction(action);

You can find more information (along with the example above) here: Calling Server-Side Actions