Avoid null artifacts when generating module IDs

src/main/java/org/apache/maven/plugins/ear/AbstractEarModule.java

@@ -334,13 +334,15 @@ protected void startModuleElement(XMLWriter writer, Boolean generateId) {
             // FIXME: Should we use the mapping using outputFileNameMapping instead
             // of doing this on our own?
             Artifact theArtifact = getArtifact();
-            String generatedId = theArtifact.getType().toUpperCase() + "_" + theArtifact.getGroupId() + "."
-                    + theArtifact.getArtifactId();
-            if (null != theArtifact.getClassifier()
-                    && !theArtifact.getClassifier().trim().isEmpty()) {
-                generatedId += "-" + theArtifact.getClassifier().trim();
+            if (theArtifact != null) {
+                String generatedId = theArtifact.getType().toUpperCase() + "_" + theArtifact.getGroupId() + "."
+                        + theArtifact.getArtifactId();
+                if (null != theArtifact.getClassifier()
+                        && !theArtifact.getClassifier().trim().isEmpty()) {
+                    generatedId += "-" + theArtifact.getClassifier().trim();
+                }
+                writer.addAttribute("id", generatedId);
             }
-            writer.addAttribute("id", generatedId);
         }
     }
 

src/test/java/org/apache/maven/plugins/ear/EarModuleTest.java

@@ -18,9 +18,15 @@
  */
 package org.apache.maven.plugins.ear;
 
+import java.io.StringWriter;
+
+import org.codehaus.plexus.util.xml.PrettyPrintXMLWriter;
+import org.codehaus.plexus.util.xml.XMLWriter;
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
 
 /**
@@ -40,4 +46,22 @@ void testCleanArchivePath() {
         assertEquals("", AbstractEarModule.cleanArchivePath(""));
         assertNull(AbstractEarModule.cleanArchivePath(null));
     }
+
+    @Test
+    void testStartModuleElementWithoutResolvedArtifact() {
+        StringWriter output = new StringWriter();
+        XMLWriter writer = new PrettyPrintXMLWriter(output);
+        TestableWebModule module = new TestableWebModule();
+
+        assertDoesNotThrow(() -> module.start(writer));
+        writer.endElement();
+
+        assertFalse(output.toString().contains("id="));
+    }
+
+    private static final class TestableWebModule extends WebModule {
+        void start(XMLWriter writer) {
+            startModuleElement(writer, true);
+        }
+    }
 }