Fix 'popupTheme' variable name in a MaterialBannerTheme method (#41632)

diff --git a/packages/flutter/lib/src/material/banner_theme.dart b/packages/flutter/lib/src/material/banner_theme.dart
index 83cc28d..7ab74c8 100644
--- a/packages/flutter/lib/src/material/banner_theme.dart
+++ b/packages/flutter/lib/src/material/banner_theme.dart
@@ -140,8 +140,8 @@
   /// MaterialBannerThemeData theme = MaterialBannerTheme.of(context);
   /// ```
   static MaterialBannerThemeData of(BuildContext context) {
-    final MaterialBannerTheme popupMenuTheme = context.inheritFromWidgetOfExactType(MaterialBannerTheme);
-    return popupMenuTheme?.data ?? Theme.of(context).bannerTheme;
+    final MaterialBannerTheme bannerTheme = context.inheritFromWidgetOfExactType(MaterialBannerTheme);
+    return bannerTheme?.data ?? Theme.of(context).bannerTheme;
   }
 
   @override