Ensure to check if editor is undefined in nb diff view (microsoft#227174)

Author: DonJayamanne

src/vs/workbench/contrib/notebook/browser/diff/diffComponents.ts

@@ -1074,7 +1074,7 @@ export class DeletedElement extends SingleSideDiffElement {
 
 	layout(state: IDiffElementLayoutState) {
 		DOM.scheduleAtNextAnimationFrame(DOM.getWindow(this._diffEditorContainer), () => {
-			if (state.editorHeight || state.outerWidth) {
+			if ((state.editorHeight || state.outerWidth) && this._editor) {
 				this._editorContainer.style.height = `${this.cell.layoutInfo.editorHeight}px`;
 				this._editor.layout({
 					width: this.cell.getComputedCellContainerWidth(this.notebookEditor.getLayoutInfo(), false, false),
@@ -1254,7 +1254,7 @@ export class InsertElement extends SingleSideDiffElement {
 
 	layout(state: IDiffElementLayoutState) {
 		DOM.scheduleAtNextAnimationFrame(DOM.getWindow(this._diffEditorContainer), () => {
-			if (state.editorHeight || state.outerWidth) {
+			if ((state.editorHeight || state.outerWidth) && this._editor) {
 				this._editorContainer.style.height = `${this.cell.layoutInfo.editorHeight}px`;
 				this._editor.layout({
 					width: this.cell.getComputedCellContainerWidth(this.notebookEditor.getLayoutInfo(), false, false),