Ensure that the viewer property, on BaseViewer-instances, is a va…

…lid div-element (issue 12320)

This should help prevent future issues, caused by the user omitting the `viewer` option and/or providing an incorrect `container` option, when initializing a `BaseViewer`-instance.

web/base_viewer.js

@@ -143,6 +143,14 @@ class BaseViewer {
 
     this.container = options.container;
     this.viewer = options.viewer || options.container.firstElementChild;
+
+    if (
+      (typeof PDFJSDev === "undefined" ||
+        PDFJSDev.test("!PRODUCTION || GENERIC")) &&
+      !(this.viewer instanceof HTMLDivElement)
+    ) {
+      throw new Error("Invalid `viewer` option.");
+    }
     this.eventBus = options.eventBus;
     this.linkService = options.linkService || new SimpleLinkService();
     this.downloadManager = options.downloadManager || null;