From 70043c3c2c28efdbbc9237b6a3b9bec36ced38a1 Mon Sep 17 00:00:00 2001
From: LostABike <22462084+LostABike@users.noreply.github.com>
Date: Wed, 10 May 2023 13:23:18 -0500
Subject: [PATCH] Add check for undefined header in Column resizer (#1156)

---
 .changeset/hip-suns-repeat.md                                | 5 +++++
 .../itwinui-react/src/core/Table/hooks/useResizeColumns.tsx  | 3 +++
 2 files changed, 8 insertions(+)
 create mode 100644 .changeset/hip-suns-repeat.md

diff --git a/.changeset/hip-suns-repeat.md b/.changeset/hip-suns-repeat.md
new file mode 100644
index 00000000000..3a3419a88e0
--- /dev/null
+++ b/.changeset/hip-suns-repeat.md
@@ -0,0 +1,5 @@
+---
+'@itwin/itwinui-react': patch
+---
+
+Fixes an issue where table crashes when resizing the Column Manager column in expand mode.
diff --git a/packages/itwinui-react/src/core/Table/hooks/useResizeColumns.tsx b/packages/itwinui-react/src/core/Table/hooks/useResizeColumns.tsx
index ccf6f733c9a..bab0eee8e78 100644
--- a/packages/itwinui-react/src/core/Table/hooks/useResizeColumns.tsx
+++ b/packages/itwinui-react/src/core/Table/hooks/useResizeColumns.tsx
@@ -494,6 +494,9 @@ function getLeafHeaders(header: HeaderGroup) {
 const getHeaderWidth = <T extends Record<string, unknown>>(
   header: ColumnInstance<T>,
 ) => {
+  if (!header) {
+    return 0;
+  }
   return Number(header.width || header.resizeWidth || 0);
 };