From 6c4bf812cda83fca6aef4e3eb41b6e8e9124b7fa Mon Sep 17 00:00:00 2001
From: Chris LaFreniere <40371649+chlafreniere@users.noreply.github.com>
Date: Wed, 20 Apr 2022 17:37:38 -0700
Subject: [PATCH] Notebooks: Preventing "previousSibling of Null" Errors on
 Cell Selection (#19153)

* Defensive programming

* PR comments
---
 .../browser/cellViews/textCell.component.ts   | 23 ++++++++++---------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/src/sql/workbench/contrib/notebook/browser/cellViews/textCell.component.ts b/src/sql/workbench/contrib/notebook/browser/cellViews/textCell.component.ts
index 9d73502445..edb992259d 100644
--- a/src/sql/workbench/contrib/notebook/browser/cellViews/textCell.component.ts
+++ b/src/sql/workbench/contrib/notebook/browser/cellViews/textCell.component.ts
@@ -212,33 +212,34 @@ export class TextCellComponent extends CellView implements OnInit, OnChanges {
 					this.cellModel.markdownCursorPosition = selection?.getPosition();
 				}
 			}
+			const selection = window.getSelection();
+			const range = selection?.rangeCount > 0 ? selection.getRangeAt(0) : undefined;
 			// On preview mode change, get the cursor position (get the position only when the selection node is a text node)
-			if (window.getSelection() && window.getSelection().focusNode?.nodeName === '#text' && window.getSelection().getRangeAt(0)) {
-				let selection = window.getSelection().getRangeAt(0);
+			if (selection.focusNode?.nodeName === '#text' && range) {
 				// Check to see if the last cursor position is still the same and skip
-				if (selection.startOffset !== this.cellModel.richTextCursorPosition?.startOffset) {
+				if (range.startOffset !== this.cellModel.richTextCursorPosition?.startOffset) {
 					// window.getSelection gives the exact html element and offsets of cursor location
 					// Since we only have the output element reference which is the parent of all html nodes
 					// we iterate through it's child nodes until we get the selection element and store the node indexes
 					// in the startElementNodes and endElementNodes and their offsets respectively.
 					let startElementNodes = [];
-					let startNode = selection.startContainer;
-					let endNode = selection.endContainer;
-					while (startNode !== this.output.nativeElement) {
+					let startNode = range.startContainer;
+					let endNode = range.endContainer;
+					while (startNode && startNode !== this.output.nativeElement) {
 						startElementNodes.push(this.getNodeIndex(startNode));
 						startNode = startNode.parentNode;
 					}
 					let endElementNodes = [];
-					while (endNode !== this.output.nativeElement) {
+					while (endNode && endNode !== this.output.nativeElement) {
 						endElementNodes.push(this.getNodeIndex(endNode));
 						endNode = endNode.parentNode;
 					}
 					// Create cursor position
 					let cursorPosition: ICaretPosition = {
 						startElementNodes: startElementNodes,
-						startOffset: selection.startOffset,
+						startOffset: range.startOffset,
 						endElementNodes: endElementNodes,
-						endOffset: selection.endOffset
+						endOffset: range.endOffset
 					};
 					this.cellModel.richTextCursorPosition = cursorPosition;
 				}
@@ -267,8 +268,8 @@ export class TextCellComponent extends CellView implements OnInit, OnChanges {
 
 	getNodeIndex(n: Node): number {
 		let i = 0;
-		// walk up the node to the top and get it's index
-		n = n.previousSibling;
+		// walk up the node to the top and get its index
+		n = n?.previousSibling;
 		while (n) {
 			i++;
 			n = n.previousSibling;