fix(Presence): fix crash on object grab if no collider is present

The Player Presence script checks to see if a grabbed object has a
collider so it can ignore collisions with grabbed objects. However,
if the grabbed object doesn't have a collider but it's children do
then it would crash. This fix prevents that and checks for child
colliders to remove the physics collisions from.

Author: thestonefox

Assets/SteamVR_Unity_Toolkit/Scripts/VRTK_PlayerPresence.cs

@@ -47,7 +47,15 @@ private void InitHeadsetListeners()
 
         private void OnGrabObject(object sender, ObjectInteractEventArgs e)
         {
-            Physics.IgnoreCollision(this.GetComponent<Collider>(), e.target.GetComponent<Collider>(), true);
+            if(e.target.GetComponent<Collider>())
+            {
+                Physics.IgnoreCollision(this.GetComponent<Collider>(), e.target.GetComponent<Collider>(), true);
+            }
+
+            foreach(var childCollider in e.target.GetComponentsInChildren<Collider>())
+            {
+                Physics.IgnoreCollision(this.GetComponent<Collider>(), childCollider, true);
+            }
         }
 
         private void OnUngrabObject(object sender, ObjectInteractEventArgs e)

Assets/SteamVR_Unity_Toolkit/Scripts/VRTK_TouchpadWalking.cs

@@ -3,6 +3,7 @@
     using UnityEngine;
     using System.Collections;
 
+    [RequireComponent(typeof(VRTK_PlayerPresence))]
     public class VRTK_TouchpadWalking : MonoBehaviour
     {
         [SerializeField]