[Subgraph] Prevent Pasting Context invalid subgraph (#191)

* Prevent pasting context within operator/block subgraph

* Fix case https://github.cds.internal.unity3d.com/unity/vfx-graphics/pull/191#discussion_r102007

Fix Approximate english log warning

* Update VFXPaste log message

Missing "the"

Author: PaulDemeulenaere

com.unity.visualeffectgraph/CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fix [Case 1290493](https://fogbugz.unity3d.com/f/cases/1290493/#BugEvent.1072735759)
 - Incorrect path on Linux while targetting Android, IOS or WebGL [Case 1279750](https://issuetracker.unity3d.com/product/unity/issues/guid/1279750/)
 - Prevent creation of subgraph containing only partial systems [Case 1284053](https://issuetracker.unity3d.com/product/unity/issues/guid/1284053/)
+- Prevent pasting context within operator/block subgraph [Case 1235269](https://issuetracker.unity3d.com/product/unity/issues/guid/1235269/)
 
 ## [10.2.0] - 2020-10-19
 ### Added

com.unity.visualeffectgraph/Editor/GraphView/Views/VFXPaste.cs

@@ -167,8 +167,21 @@ void PasteAll(VFXViewController viewController, Vector2 center, ref Serializable
             pasteOffset = (serializableGraph.bounds.width > 0 && serializableGraph.bounds.height > 0) ? center - serializableGraph.bounds.center : Vector2.zero;
             MakePasteOffsetUnique(viewController, serializableGraph);
 
-            // Paste all nodes
-            PasteContexts(viewController, ref serializableGraph);
+            // Can't paste context within subgraph block/operator
+            if (viewController.model.visualEffectObject is VisualEffectSubgraphOperator || viewController.model.visualEffectObject is VisualEffectSubgraphBlock)
+            {
+                if (serializableGraph.contexts != null)
+                {
+                    var count = serializableGraph.contexts.Count();
+                    if (count != 0)
+                        Debug.LogWarningFormat("{0} context{1} been skipped during the paste operation. Contexts aren't available in this kind of subgraph.", count, count > 1 ? "s have" : " has");
+                }
+            }
+            else
+            {
+                PasteContexts(viewController, ref serializableGraph);
+            }
+
             PasteOperators(viewController, ref serializableGraph);
             PasteParameters(viewController, ref serializableGraph);
 
@@ -626,7 +639,7 @@ private void RegisterContexts(VFXViewController viewController)
                         var block = newContexts[i].Value[j];
                         if (block != null)
                         {
-                            VFXBlockController blockController = controller.blockControllers.First(t => t.model == block);
+                            VFXBlockController blockController = controller.blockControllers.FirstOrDefault(t => t.model == block);
                             if (blockController != null)
                                 newControllers[GetBlockID((uint)i, (uint)j)] = blockController;
                         }