Merge pull request #247 from keijiro/fix-null-preview

Fix for NullReferenceException in master preview.

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Inspector/MasterPreviewView.cs

 
         IMasterNode masterNode
         {
-            get { return m_PreviewRenderHandle.shaderData.node as IMasterNode; }
+            get { return m_PreviewRenderHandle.shaderData != null ? m_PreviewRenderHandle.shaderData.node as IMasterNode : null; }
         }
 
         void DirtyMasterNode(ModificationScope scope)

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/PreviewManager.cs

         public void ResizeMasterPreview(Vector2 newSize)
         {
             m_NewMasterPreviewSize = newSize;
-            m_DirtyPreviews.Add(masterRenderData.shaderData.node.tempId.index);
+            if (masterRenderData.shaderData != null)
+                m_DirtyPreviews.Add(masterRenderData.shaderData.node.tempId.index);
         }
 
         public PreviewRenderData GetPreview(AbstractMaterialNode node)