fix null check in links

src/vs/editor/contrib/links/links.ts

@@ -341,7 +341,7 @@ class LinkDetector implements editorCommon.IEditorContribution {
 		}, err => {
 			// different error cases
 			if (err === 'invalid') {
-				this.notificationService.warn(nls.localize('invalid.url', 'Failed to open this link because it is not well-formed: {0}', link.url.toString()));
+				this.notificationService.warn(nls.localize('invalid.url', 'Failed to open this link because it is not well-formed: {0}', link.url!.toString()));
 			} else if (err === 'missing') {
 				this.notificationService.warn(nls.localize('missing.url', 'Failed to open this link because its target is missing.'));
 			} else {