[FIX] Fixed an issue where openening up an empty shadergraph without any nodes caused an error.

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

 
         void DestroyRenderData(PreviewRenderData renderData)
         {
-            if (renderData.shaderData.shader != null && renderData.shaderData.shader != m_UberShader)
+            if (renderData.shaderData != null
+                && renderData.shaderData.shader != null 
+                && renderData.shaderData.shader != m_UberShader)
-            var node = renderData.shaderData.node;
-            if (node != null)
-                node.onModified -= OnNodeModified;
+            
+            if (renderData.shaderData != null && renderData.shaderData.node != null)
+                renderData.shaderData.node.onModified -= OnNodeModified;
         }
 
         void DestroyPreview(Identifier nodeId)

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Views/GraphEditorView.cs

 
         public void Dispose()
         {
-            saveRequested = null;
-            convertToSubgraphRequested = null;
-            showInProjectRequested = null;
+                saveRequested = null;
+                convertToSubgraphRequested = null;
+                showInProjectRequested = null;
                 foreach (var node in m_GraphView.Children().OfType<MaterialNodeView>())
                     node.Dispose();
                 m_GraphView = null;