Add a null check to areSelectionValuesReversed

See Microsoft/vscode#35601

src/SelectionModel.ts

@@ -97,6 +97,9 @@ export class SelectionModel {
   public areSelectionValuesReversed(): boolean {
     const start = this.selectionStart;
     const end = this.selectionEnd;
+    if (!start || !end) {
+      return false;
+    }
     return start[1] > end[1] || (start[1] === end[1] && start[0] > end[0]);
   }