fix #51667

src/vs/editor/contrib/snippet/snippetSession.ts

@@ -182,6 +182,14 @@ export class OneSnippet {
 
 				const id = this._placeholderDecorations.get(placeholder);
 				const range = this._editor.getModel().getDecorationRange(id);
+				if (!range) {
+					// one of the placeholder lost its decoration and
+					// therefore we bail out and pretend the placeholder
+					// (with its mirrors) doesn't exist anymore.
+					result.delete(placeholder.index);
+					break;
+				}
+
 				ranges.push(range);
 			}
 		}