[Editor] Don't throw when switching to editing mode before an annotation editor layer is rendered

It can happen with a pdf having a large text layer.
Instead of waiting for the first rendered page to enable the buttons we wait for
a rendered annotation editor layer.

web/pdf_viewer.js

@@ -1072,6 +1072,20 @@ class PDFViewer {
           this._updateSpreadMode();
         }
 
+        eventBus._on(
+          "annotationeditorlayerrendered",
+          evt => {
+            if (this.#annotationEditorUIManager) {
+              // Ensure that the Editor buttons, in the toolbar, are updated.
+              eventBus.dispatch("annotationeditormodechanged", {
+                source: this,
+                mode: this.#annotationEditorMode,
+              });
+            }
+          },
+          { once: true, signal }
+        );
+
         // Fetch all the pages since the viewport is needed before printing
         // starts to create the correct size canvas. Wait until one page is
         // rendered so we don't tie up too many resources early on.
@@ -1090,14 +1104,6 @@ class PDFViewer {
             );
           }
 
-          if (this.#annotationEditorUIManager) {
-            // Ensure that the Editor buttons, in the toolbar, are updated.
-            eventBus.dispatch("annotationeditormodechanged", {
-              source: this,
-              mode: this.#annotationEditorMode,
-            });
-          }
-
           // In addition to 'disableAutoFetch' being set, also attempt to reduce
           // resource usage when loading *very* long/large documents.
           if (