Add check for undefined header in Column resizer

.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.

packages/itwinui-react/src/core/Table/hooks/useResizeColumns.tsx

@@ -496,6 +496,9 @@ 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);
 };