Fix MockitoMockMaker throws NPE on null object (#2338)

MockMakerRegistry.asMockOrNull() shall return null on null object and
not call the mock makers.

Fixes #2337

Author: AndreasTu

docs/release_notes.adoc

@@ -14,6 +14,7 @@ include::include.adoc[]
 
 * Fix argument mismatch descriptions for varargs methods by expanding varargs instead of reporting `<too few arguments>` spockPull:2315[]
 * Fix Pattern flags being dropped when `java.util.regex.Pattern` instances are used in Spock regex conditions spockIssue:2298[]
+* Fix `MockitoMockMaker` throws NPE on null object spockIssue:2337[]
 
 == 2.4 (2025-12-11)
 

spock-core/src/main/java/org/spockframework/mock/runtime/IMockMaker.java

@@ -125,6 +125,7 @@ default IStaticMock makeStaticMock(IMockCreationSettings settings) throws Cannot
    * @param object a potential mock object
    * @return information about the mock object or {@code null}
    */
+  @Nullable
   default IMockObject asMockOrNull(Object object) {
     return null;
   }

spock-core/src/main/java/org/spockframework/mock/runtime/MockMakerRegistry.java

@@ -22,7 +22,6 @@
 import org.spockframework.mock.ISpockMockObject;
 import org.spockframework.mock.runtime.IMockMaker.IStaticMock;
 import org.spockframework.mock.runtime.IMockMaker.MockMakerCapability;
-import org.spockframework.runtime.GroovyRuntimeUtil;
 import org.spockframework.util.InternalSpockError;
 import org.spockframework.util.Nullable;
 import org.spockframework.util.ThreadSafe;
@@ -182,7 +181,11 @@ private static void checkForStaticMockUsageWithClosure(IMockCreationSettings set
    * @param object a mock object
    * @return information about the mock object
    */
-  public IMockObject asMockOrNull(Object object) {
+  @Nullable
+  public IMockObject asMockOrNull(@Nullable Object object) {
+    if (object == null) {
+      return null;
+    }
     if (object instanceof ISpockMockObject) {
       return ((ISpockMockObject) object).$spock_get();
     }

spock-core/src/main/java/org/spockframework/mock/runtime/mockito/MockitoMockMaker.java

@@ -19,6 +19,7 @@
 import org.spockframework.mock.CannotCreateMockException;
 import org.spockframework.mock.IMockObject;
 import org.spockframework.mock.runtime.IMockMaker;
+import org.spockframework.util.Nullable;
 import org.spockframework.util.ReflectionUtil;
 import org.spockframework.util.ThreadSafe;
 import spock.mock.MockMakerId;
@@ -69,8 +70,9 @@ public int getPriority() {
   }
 
   @Override
-  public IMockObject asMockOrNull(Object object) {
-    if (impl == null) {
+  @Nullable
+  public IMockObject asMockOrNull(@Nullable Object object) {
+    if (impl == null || object == null) {
       return null;
     }
     return impl.asMockOrNull(object);

spock-specs/src/test/groovy/org/spockframework/mock/runtime/mockito/MockitoMockMakerSpec.groovy

@@ -45,6 +45,14 @@ class MockitoMockMakerSpec extends Specification {
     mockito.toString() == "$ID default mock maker settings"
   }
 
+  def "MockitoMockMaker.asMockOrNull() on null returns null"() {
+    given:
+    def maker = new MockitoMockMaker()
+
+    expect:
+    maker.asMockOrNull(null) == null
+  }
+
   def "Verify ID and IMockMakerSettings with Mockito settings"() {
     when:
     def set = mockito {}
@@ -507,6 +515,19 @@ Can not mock final classes with the following settings :
     additionalInterfaceClass.isInstance(m)
     mockUtil.isMock(m)
   }
+
+  @Issue("https://github.com/spockframework/spock/issues/2337")
+  def "NPE when stubbing method on null object"() {
+    given:
+    Object nullObj = null
+
+    when:
+    //Issue #2337: MockitoMockMaker passes null to Mockito's getHandler() throws NPE
+    nullObj.toString() >> ""
+
+    then:
+    noExceptionThrown()
+  }
 }
 
 class AccessProtectedBaseClass {