fixed joint crash when no lead entity is selected (#2867)

Signed-off-by: amzn-sean <75276488+amzn-sean@users.noreply.github.com>

Gems/PhysX/Code/Source/BallJointComponent.cpp

@@ -46,11 +46,26 @@ namespace PhysX
 
         JointComponent::LeadFollowerInfo leadFollowerInfo;
         ObtainLeadFollowerInfo(leadFollowerInfo);
-        if (!leadFollowerInfo.m_followerActor)
+        if (leadFollowerInfo.m_followerActor == nullptr ||
+            leadFollowerInfo.m_followerBody == nullptr)
         {
             return;
         }
 
+        // if there is no lead body, this will be a constraint of the follower's global position, so use invalid body handle.
+        AzPhysics::SimulatedBodyHandle parentHandle = AzPhysics::InvalidSimulatedBodyHandle;
+        if (leadFollowerInfo.m_leadBody != nullptr)
+        {
+            parentHandle = leadFollowerInfo.m_leadBody->m_bodyHandle;
+        }
+        else
+        {
+            AZ_TracePrintf(
+                "PhysX", "Entity [%s] Ball Joint component missing lead entity. This joint will be a global constraint on the follower's global position.",
+                GetEntity()->GetName().c_str());
+        }
+        
+
         BallJointConfiguration configuration;
         configuration.m_parentLocalPosition = leadFollowerInfo.m_leadLocal.GetTranslation();
         configuration.m_parentLocalRotation = leadFollowerInfo.m_leadLocal.GetRotation();
@@ -65,7 +80,7 @@ namespace PhysX
             m_jointHandle = sceneInterface->AddJoint(
                 leadFollowerInfo.m_followerBody->m_sceneOwner,
                 &configuration,  
-                leadFollowerInfo.m_leadBody->m_bodyHandle, 
+                parentHandle,
                 leadFollowerInfo.m_followerBody->m_bodyHandle);
             m_jointSceneOwner = leadFollowerInfo.m_followerBody->m_sceneOwner;
         }