diff --git a/src/resizer/distributors/collapse.ts b/src/resizer/distributors/collapse.ts
index 96d355d326..507b62e903 100644
--- a/src/resizer/distributors/collapse.ts
+++ b/src/resizer/distributors/collapse.ts
@@ -21,7 +21,7 @@ import Sizer from "../sizer";
 
 export interface ICollapseConfig extends IConfig {
     toggleSize: number;
-    onCollapsed?(collapsed: boolean, id: string, element: HTMLElement): void;
+    onCollapsed?(collapsed: boolean, id: string | null, element: HTMLElement): void;
     isItemCollapsed(element: HTMLElement): boolean;
 }
 
@@ -45,7 +45,7 @@ export default class CollapseDistributor extends FixedDistributor<ICollapseConfi
         return new CollapseItem(resizeHandle, resizer, sizer, container);
     }
 
-    private readonly toggleSize: number;
+    private readonly toggleSize: number | undefined;
     private isCollapsed: boolean;
 
     public constructor(item: CollapseItem) {
@@ -55,7 +55,7 @@ export default class CollapseDistributor extends FixedDistributor<ICollapseConfi
     }
 
     public resize(newSize: number): void {
-        const isCollapsedSize = newSize < this.toggleSize;
+        const isCollapsedSize = !!this.toggleSize && newSize < this.toggleSize;
         if (isCollapsedSize !== this.isCollapsed) {
             this.isCollapsed = isCollapsedSize;
             this.item.notifyCollapsed(isCollapsedSize);
diff --git a/src/resizer/distributors/percentage.ts b/src/resizer/distributors/percentage.ts
index 28e29f4068..51dd9b6eca 100644
--- a/src/resizer/distributors/percentage.ts
+++ b/src/resizer/distributors/percentage.ts
@@ -21,9 +21,9 @@ import { IConfig } from "../resizer";
 class PercentageSizer extends Sizer {
     public start(item: HTMLElement): void {
         if (this.vertical) {
-            item.style.minHeight = null;
+            item.style.minHeight = "";
         } else {
-            item.style.minWidth = null;
+            item.style.minWidth = "";
         }
     }
 
diff --git a/src/resizer/item.ts b/src/resizer/item.ts
index 7c4b8645fc..6c979225d6 100644
--- a/src/resizer/item.ts
+++ b/src/resizer/item.ts
@@ -44,14 +44,16 @@ export default class ResizeItem<C extends IConfig = IConfig> {
 
     private advance(forwards: boolean): ResizeItem | undefined {
         // opposite direction from fromResizeHandle to get back to handle
-        let handle = this.reverse ? this.domNode.previousElementSibling : this.domNode.nextElementSibling;
+        let handle: Element | null | undefined = this.reverse
+            ? this.domNode.previousElementSibling
+            : this.domNode.nextElementSibling;
         const moveNext = forwards !== this.reverse; // xor
         // iterate at least once to avoid infinite loop
         do {
             if (moveNext) {
-                handle = handle.nextElementSibling;
+                handle = handle?.nextElementSibling;
             } else {
-                handle = handle.previousElementSibling;
+                handle = handle?.previousElementSibling;
             }
         } while (handle && !this.resizer.isResizeHandle(<HTMLElement>handle));
 
@@ -105,6 +107,9 @@ export default class ResizeItem<C extends IConfig = IConfig> {
     }
 
     public first(): ResizeItem | undefined {
+        if (!this.domNode.parentElement?.children) {
+            return;
+        }
         const firstHandle = Array.from(this.domNode.parentElement.children).find((el) => {
             return this.resizer.isResizeHandle(<HTMLElement>el);
         });
@@ -114,6 +119,9 @@ export default class ResizeItem<C extends IConfig = IConfig> {
     }
 
     public last(): ResizeItem | undefined {
+        if (!this.domNode.parentElement?.children) {
+            return;
+        }
         const lastHandle = Array.from(this.domNode.parentElement.children)
             .reverse()
             .find((el) => {