use ref when dealing with cell text model.

src/vs/workbench/contrib/notebook/browser/viewModel/baseCellViewModel.ts

@@ -104,7 +104,7 @@ export abstract class BaseCellViewModel extends Disposable {
 	}
 
 	hasModel(): this is IEditableCellViewModel {
-		return !!this.model.textModel;
+		return !!this._textModel;
 	}
 
 	private _dragging: boolean = false;

src/vs/workbench/contrib/notebook/common/model/notebookCellTextModel.ts

@@ -70,15 +70,6 @@ export class NotebookCellTextModel extends Disposable implements ICell {
 		return this._textBuffer;
 	}
 
-	private _textModel?: model.ITextModel;
-
-	get textModel(): model.ITextModel | undefined {
-		return this._textModel;
-	}
-
-	set textModel(m: model.ITextModel | undefined) {
-		this._textModel = m;
-	}
 
 	constructor(
 		readonly uri: URI,
@@ -144,10 +135,6 @@ export class NotebookCellTextModel extends Disposable implements ICell {
 
 	async resolveTextModelRef() {
 		const ref = await this._modelService.createModelReference(this.uri);
-
-		ref.object.textEditorModel.onWillDispose(() => {
-			this.textModel = undefined;
-		});
 		return ref;
 	}
 

src/vs/workbench/contrib/notebook/common/model/notebookTextModel.ts

@@ -559,14 +559,15 @@ export class NotebookTextModel extends Disposable implements INotebookTextModel
 	async splitNotebookCell(index: number, newLinesContents: string[], endSelections: number[]) {
 		const cell = this.cells[index];
 
-		if (!cell.textModel) {
-			return;
-		}
+		const ref = await cell.resolveTextModelRef();
+		const textModel = ref.object.textEditorModel;
 
-		cell.textModel.applyEdits([
-			{ range: cell.textModel.getFullModelRange(), text: newLinesContents[0] }
+		textModel.applyEdits([
+			{ range: textModel.getFullModelRange(), text: newLinesContents[0] }
 		], false);
 
+		ref.dispose();
+
 		// create new cells based on the new text models
 		const language = cell.language;
 		const kind = cell.cellKind;