Fixed NPE in FindReturnRef on inner class fields (spotbugs#3283)

Fixes spotbugs#3283

Author: iloveeclipse

CHANGELOG.md

@@ -13,6 +13,7 @@ Currently the versioning policy of this project follows [Semantic Versioning v2.
 - Fixed the an analysis error when `FindReturnRef` was checking instructions corresponding to a CFG branch that was optimized away ([#3266](https://github.com/spotbugs/spotbugs/issues/3266))
 - Added execute file permission to files in the distribution archive ([#3274](https://github.com/spotbugs/spotbugs/issues/3274))
 - Fixed a stack overflow in  `MultipleInstantiationsOfSingletons` when a singleton initializer makes recursive calls ([#3280](https://github.com/spotbugs/spotbugs/issues/3280))
+- Fixed NPE in  `FindReturnRef` on inner class fields ([#3283](https://github.com/spotbugs/spotbugs/issues/3283))
 
 ## 4.9.0 - 2025-01-15
 ### Added

spotbugs/src/main/java/edu/umd/cs/findbugs/detect/FindReturnRef.java

@@ -315,7 +315,7 @@ public void sawOpcode(int seen) {
     }
 
     private boolean isNestedField(XField field) {
-        if (getThisClass().isNested() && field.getName().startsWith("this$")) {
+        if (field != null && getThisClass().isNested() && field.getName().startsWith("this$")) {
             try {
                 List<JavaClass> hostClasses = NestedAccessUtil.getHostClasses(getThisClass());
                 String fieldType = ClassName.fromFieldSignatureToDottedClassName(field.getSignature());

spotbugsTestCases/src/java/exposemutable/FindReturnRefNegativeTest.java

@@ -183,6 +183,16 @@ private void accessParent() {
         }
     }
 
+	public static class PreferenceTreeNode {
+		private PreferenceTreeNode fParent; 
+		public void addChild(PreferenceTreeNode node) {
+			node.fParent= this; 
+		}
+		public PreferenceTreeNode getfParent() {
+			return fParent;
+		}
+	} 
+    
     private CharBuffer charBuf;
     private char[] charArray;