[SalesForce] get apex controller value in lightning component controller

You need to give the attribute a default value:

<aura:attribute name="listItem" type="ToDoObj__c" default="{ sobjectType: 'ToDoObj__c', Name: '' }"/>

There is a unit here in Trailhead that shows a simple example, code below:

Apex controller:

public with sharing class SimpleServerSideController {

    //Use @AuraEnabled to enable client- and server-side access to the method
    @AuraEnabled
    public static String serverEcho(String firstName) {
        return ('Hello from the server, ' + firstName);
    }
}

Lightning Component

<aura:component controller="SimpleServerSideController">
    <aura:attribute name="firstName" type="String" default="world"/>
    <ui:button label="Call server" press="{!c.echo}"/>
</aura:component>

Client-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();
            // This callback doesn’t reference cmp. If it did,
            // you should run an isValid() check
            //if (cmp.isValid() && state === "SUCCESS") {
            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 (cmp.isValid() && state === "INCOMPLETE") {
            else if (state === "INCOMPLETE") {
                // do something
            }
            //else if (cmp.isValid() && state === "ERROR") {
            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);
    }
})