check-flagged-apis: add missing requireNotNull

Bug: 334870672
Test: atest --host check-flagged-apis-test
Change-Id: Ibffafb504240c82d525105d4678983b30b70592d
diff --git a/tools/check-flagged-apis/src/com/android/checkflaggedapis/Main.kt b/tools/check-flagged-apis/src/com/android/checkflaggedapis/Main.kt
index 84564ba..c4c5b11 100644
--- a/tools/check-flagged-apis/src/com/android/checkflaggedapis/Main.kt
+++ b/tools/check-flagged-apis/src/com/android/checkflaggedapis/Main.kt
@@ -207,7 +207,10 @@
   // ktfmt doesn't understand the `..<` range syntax; explicitly call .rangeUntil instead
   for (i in 0.rangeUntil(fields.getLength())) {
     val field = fields.item(i)
-    val fieldName = field.getAttribute("name")
+    val fieldName =
+        requireNotNull(field.getAttribute("name")) {
+          "Bad XML: <field> element without name attribute"
+        }
     val className =
         requireNotNull(field.getParentNode()) { "Bad XML: top level <field> element" }
             .getAttribute("name")