Fail loudly when LayoutWrappingEncoder has no layout (#1046)

logback-core/src/main/java/ch/qos/logback/core/encoder/LayoutWrappingEncoder.java

@@ -111,6 +111,9 @@ private byte[] convertToBytes(String s) {
     }
 
     public byte[] encode(E event) {
+        if (layout == null) {
+            return null;
+        }
         String txt = layout.doLayout(event);
         return convertToBytes(txt);
     }
@@ -120,6 +123,10 @@ public boolean isStarted() {
     }
 
     public void start() {
+        if (layout == null) {
+            addError("No layout set for the encoder. This encoder will produce no output. "
+                    + "Please set a layout, or check for an ignored <if>/<then>/<else> block in your configuration.");
+        }
         if (immediateFlush != null) {
             if (parent instanceof OutputStreamAppender) {
                 addWarn("Setting the \"immediateFlush\" property of the enclosing appender to " + immediateFlush);

logback-core/src/test/java/ch/qos/logback/core/encoder/LayoutWrappingEncoderTest.java

@@ -0,0 +1,67 @@
+/*
+ * Logback: the reliable, generic, fast and flexible logging framework.
+ * Copyright (C) 1999-2026, QOS.ch. All rights reserved.
+ *
+ * This program and the accompanying materials are dual-licensed under
+ * either the terms of the Eclipse Public License v2.0 as published by
+ * the Eclipse Foundation
+ *
+ *   or (per the licensee's choosing)
+ *
+ * under the terms of the GNU Lesser General Public License version 2.1
+ * as published by the Free Software Foundation.
+ */
+package ch.qos.logback.core.encoder;
+
+import ch.qos.logback.core.Context;
+import ch.qos.logback.core.ContextBase;
+import ch.qos.logback.core.LayoutBase;
+import ch.qos.logback.core.status.Status;
+import ch.qos.logback.core.status.testUtil.StatusChecker;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+public class LayoutWrappingEncoderTest {
+
+    Context context = new ContextBase();
+    StatusChecker statusChecker = new StatusChecker(context);
+    LayoutWrappingEncoder<Object> encoder = new LayoutWrappingEncoder<Object>();
+
+    static class EchoLayout extends LayoutBase<Object> {
+        public String doLayout(Object event) {
+            return String.valueOf(event);
+        }
+    }
+
+    @BeforeEach
+    public void setUp() {
+        encoder.setContext(context);
+    }
+
+    // https://github.com/qos-ch/logback/issues/1046
+    // A wrapping encoder left without a layout (e.g. when an <if>/<then>/<else>
+    // block is silently ignored) used to start "successfully" and then throw a
+    // NullPointerException on every logging event, causing silent log loss.
+    @Test
+    public void nullLayoutReportsErrorOnStartAndDoesNotThrowOnEncode() {
+        encoder.start();
+        statusChecker.assertContainsMatch(Status.ERROR, "No layout set for the encoder");
+
+        byte[] result = assertDoesNotThrow(() -> encoder.encode(new Object()));
+        assertNull(result);
+    }
+
+    @Test
+    public void encodesWhenLayoutIsSet() {
+        encoder.setLayout(new EchoLayout());
+        encoder.start();
+        statusChecker.assertIsErrorFree();
+
+        byte[] result = encoder.encode("hello");
+        assertEquals("hello", new String(result));
+    }
+}