diff --git a/src/diff/base-unified-diff.ts b/src/diff/base-unified-diff.ts
index e694f9a..cd899a6 100644
--- a/src/diff/base-unified-diff.ts
+++ b/src/diff/base-unified-diff.ts
@@ -69,7 +69,7 @@ export abstract class BaseUnifiedDiffManager {
       return;
     }
     this._isDisposed = true;
-    this._deactivate();
+    this.deactivate();
   }
 
   /**
@@ -112,7 +112,7 @@ export abstract class BaseUnifiedDiffManager {
   /**
    * Deactivate the diff view
    */
-  protected _deactivate(): void {
+  protected deactivate(): void {
     this.removeToolbarButtons();
     this._cleanupEditor();
     this.showCellToolbar();
@@ -137,7 +137,7 @@ export abstract class BaseUnifiedDiffManager {
    */
   protected acceptAll(): void {
     // simply accept the current state
-    this._deactivate();
+    this.deactivate();
   }
 
   /**
@@ -146,7 +146,7 @@ export abstract class BaseUnifiedDiffManager {
   protected rejectAll(): void {
     const sharedModel = this.getSharedModel();
     sharedModel.setSource(this._originalSource);
-    this._deactivate();
+    this.deactivate();
   }
 
   /**
@@ -166,7 +166,7 @@ export abstract class BaseUnifiedDiffManager {
       newSource: this._newSource,
       isInitialized: this._isInitialized,
       sharedModel: this.getSharedModel(),
-      onChunkChange: () => this._deactivate()
+      onChunkChange: () => this.deactivate()
     });
 
     this._isInitialized = true;
diff --git a/src/diff/unified-cell.ts b/src/diff/unified-cell.ts
index db7840b..b98cc2a 100644
--- a/src/diff/unified-cell.ts
+++ b/src/diff/unified-cell.ts
@@ -37,6 +37,7 @@ export class UnifiedCellDiffManager extends BaseUnifiedDiffManager {
   }
 
   private static _activeDiffCount = 0;
+  private _toolbarObserver?: MutationObserver;
 
   /**
    * Get the shared model for source manipulation
@@ -61,29 +62,32 @@ export class UnifiedCellDiffManager extends BaseUnifiedDiffManager {
       subtree: true
     });
 
-    (this as any)._toolbarObserver = observer;
+    this._toolbarObserver = observer;
   }
 
   /**
    * Deactivate the diff view with cell toolbar.
    */
-  protected _deactivate(): void {
-    super['_deactivate']();
+  protected deactivate(): void {
+    super.deactivate();
     UnifiedCellDiffManager._activeDiffCount = Math.max(
       0,
       UnifiedCellDiffManager._activeDiffCount - 1
     );
 
-    const observer = (this as any)._toolbarObserver as MutationObserver;
-    if (observer) {
-      observer.disconnect();
+    if (this._toolbarObserver) {
+      this._toolbarObserver.disconnect();
+      this._toolbarObserver = undefined;
     }
   }
+
   /**
    * Hide the cell's toolbar while the diff is active
    */
   protected hideCellToolbar(): void {
-    const toolbar = this._cell.node.querySelector('jp-toolbar') as HTMLElement;
+    const toolbar = this._cell.node.querySelector(
+      'jp-toolbar'
+    ) as HTMLElement | null;
     if (toolbar) {
       toolbar.style.display = 'none';
     }