fix(combobox): Fix error when typing a custom value (#6071)

**Related Issue:** #5109

## Summary

fix(combobox): Fix error when typing a custom value and then trying to
open the menu. #5109

src/components/combobox/combobox.e2e.ts

@@ -586,6 +586,20 @@ describe("calcite-combobox", () => {
       expect(floatingUI).toBeNull();
     });
 
+    it("should not throw when typing custom value and pressing ArrowDown", async () => {
+      const combobox = await page.find("calcite-combobox");
+      combobox.setProperty("allowCustomValues", true);
+      await page.waitForChanges();
+      const inputEl = await page.find(`#myCombobox >>> input`);
+      await inputEl.focus();
+      await page.waitForChanges();
+      expect(await page.evaluate(() => document.activeElement.id)).toBe("myCombobox");
+      await page.keyboard.type("asdf");
+      await page.waitForChanges();
+      await page.keyboard.press("ArrowDown");
+      await page.waitForChanges();
+    });
+
     it(`ArrowDown opens the item group for combobox in focus and jumps to the first item`, async () => {
       const inputEl = await page.find(`#myCombobox >>> input`);
       await inputEl.focus();

src/components/combobox/combobox.tsx

@@ -748,6 +748,10 @@ export class Combobox
   }
 
   private calculateSingleItemHeight(item: ComboboxChildElement): number {
+    if (!item) {
+      return;
+    }
+
     let height = item.offsetHeight;
     // if item has children items, don't count their height twice
     const children = Array.from(item.querySelectorAll<ComboboxChildElement>(ComboboxChildSelector));
@@ -1003,6 +1007,11 @@ export class Combobox
 
   private scrollToActiveItem = (): void => {
     const activeItem = this.filteredItems[this.activeItemIndex];
+
+    if (!activeItem) {
+      return;
+    }
+
     const height = this.calculateSingleItemHeight(activeItem);
     const { offsetHeight, scrollTop } = this.listContainerEl;
     if (offsetHeight + scrollTop < activeItem.offsetTop + height) {