Fix sidebar container initialization when "clean" theme is used

When the "clean" theme is used, the sidebar's toggle button is not created and
constructing the `DragHandler` failed because it was passed a `null` target.
Update the types in `ToolbarController.sidebarToggleButton` to reflect that the
value can be null and handle this in `Sidebar` by not initializing
`_dragResizeHandler` in this case.

src/annotator/sidebar.tsx

@@ -105,7 +105,7 @@ type DragResizeState = {
 export class Sidebar implements Destroyable {
   private _emitter: Emitter;
   private _config: SidebarContainerConfig & SidebarConfig;
-  private _dragResizeHandler: DragHandler;
+  private _dragResizeHandler: DragHandler | undefined;
   private _dragResizeState: DragResizeState;
   private _listeners: ListenerCollection;
   private _layoutState: SidebarLayout;
@@ -292,10 +292,15 @@ export class Sidebar implements Destroyable {
       initial: null,
       final: null,
     };
-    this._dragResizeHandler = new DragHandler({
-      target: this.toolbar.sidebarToggleButton,
-      onDrag: event => this._onDragSidebarToggleButton(event),
-    });
+
+    const toggleButton = this.toolbar.sidebarToggleButton;
+    if (toggleButton) {
+      this._dragResizeHandler = new DragHandler({
+        target: toggleButton,
+        onDrag: event => this._onDragSidebarToggleButton(event),
+      });
+    }
+
     this.close();
 
     // Publisher-provided callback functions
@@ -327,7 +332,7 @@ export class Sidebar implements Destroyable {
     this._sidebarRPC.destroy();
     this.bucketBar?.destroy();
     this._listeners.removeAll();
-    this._dragResizeHandler.destroy();
+    this._dragResizeHandler?.destroy();
     if (this._hypothesisSidebar) {
       // Explicitly unmounting the "messages" element, to make sure effects are clean-up
       render(null, this._messagesElement!);

src/annotator/test/sidebar-test.js

@@ -154,15 +154,26 @@ describe('Sidebar', () => {
     fakeDragHandler = {
       destroy: sinon.stub(),
     };
-    FakeDragHandler = sinon.stub().returns(fakeDragHandler);
+    FakeDragHandler = sinon.stub().callsFake(options => {
+      assert.isNotNull(options.target);
+      assert.isFunction(options.onDrag);
+      return fakeDragHandler;
+    });
 
+    const toggleButton = document.createElement('button');
     fakeToolbar = {
       getWidth: sinon.stub().returns(100),
       useMinimalControls: false,
       sidebarOpen: false,
       newAnnotationType: 'note',
       highlightsVisible: false,
-      sidebarToggleButton: document.createElement('button'),
+      get sidebarToggleButton() {
+        if (this.useMinimalControls) {
+          return null;
+        } else {
+          return toggleButton;
+        }
+      },
     };
     FakeToolbarController = sinon.stub().returns(fakeToolbar);
 

src/annotator/toolbar.tsx

@@ -27,7 +27,7 @@ export class ToolbarController {
   private _toggleSidebar: () => void;
   private _toggleHighlights: () => void;
   private _createAnnotation: () => void;
-  private _sidebarToggleButton: RefObject<HTMLElement>;
+  private _sidebarToggleButton: RefObject<HTMLButtonElement>;
 
   /**
    * @param container - Element into which the toolbar is rendered
@@ -52,7 +52,7 @@ export class ToolbarController {
     };
 
     /** Reference to the sidebar toggle button. */
-    this._sidebarToggleButton = createRef<HTMLElement>();
+    this._sidebarToggleButton = createRef();
 
     this.render();
   }
@@ -115,9 +115,11 @@ export class ToolbarController {
 
   /**
    * Return the DOM element that toggles the sidebar's visibility.
+   *
+   * This will be `null` if {@link useMinimalControls} is true.
    */
-  get sidebarToggleButton() {
-    return this._sidebarToggleButton.current as HTMLButtonElement;
+  get sidebarToggleButton(): HTMLButtonElement | null {
+    return this._sidebarToggleButton.current;
   }
 
   render() {