fix: UnsupportedOperationException on adding a field (#1645)(PR #1646)

* fix: UnsupportedOperationException on adding a field

* changed list check and creation similar to safeAdd

jadx-core/src/main/java/jadx/core/dex/nodes/ClassNode.java

@@ -469,6 +469,9 @@ public class ClassNode extends NotificationAttrNode implements ILoadable, ICodeN
 	}
 
 	public void addField(FieldNode fld) {
+		if (fields == null || fields.isEmpty()) {
+			fields = new ArrayList<>(1);
+		}
 		fields.add(fld);
 	}
 

jadx-core/src/main/java/jadx/core/utils/android/AndroidResourcesUtils.java

@@ -129,7 +129,7 @@ public class AndroidResourcesUtils {
 				FieldNode newResField = new FieldNode(typeCls, rFieldInfo,
 						AccessFlags.PUBLIC | AccessFlags.STATIC | AccessFlags.FINAL);
 				newResField.addAttr(new EncodedValue(EncodedType.ENCODED_INT, resource.getId()));
-				typeCls.getFields().add(newResField);
+				typeCls.addField(newResField);
 				if (rClsExists) {
 					newResField.addInfoComment("Added by JADX");
 				}