Strict null check

src/tsconfig.strictNullChecks.json

@@ -333,6 +333,7 @@
 		"./vs/editor/contrib/parameterHints/provideSignatureHelp.ts",
 		"./vs/editor/contrib/quickOpen/quickOpen.ts",
 		"./vs/editor/contrib/referenceSearch/referencesModel.ts",
+		"./vs/editor/contrib/referenceSearch/referencesTree.ts",
 		"./vs/editor/contrib/rename/rename.ts",
 		"./vs/editor/contrib/rename/renameInputField.ts",
 		"./vs/editor/contrib/smartSelect/smartSelect.ts",

src/vs/editor/contrib/referenceSearch/referencesTree.ts

@@ -166,10 +166,14 @@ class OneReferenceTemplate {
 	}
 
 	set(element: OneReference): void {
-		const { before, inside, after } = element.parent.preview.preview(element.range);
-		this.before.innerHTML = escape(before);
-		this.inside.innerHTML = escape(inside);
-		this.after.innerHTML = escape(after);
+		const filePreview = element.parent.preview;
+		const preview = filePreview && filePreview.preview(element.range);
+		if (preview) {
+			const { before, inside, after } = preview;
+			this.before.innerHTML = escape(before);
+			this.inside.innerHTML = escape(inside);
+			this.after.innerHTML = escape(after);
+		}
 	}
 }