Made the implementation null-safe

src/main/java/org/shipkit/changelog/GenerateChangelogTask.java

@@ -144,15 +144,20 @@ public File getWorkingDir() {
         return workingDir;
     }
 
+    @Optional
     @InputDirectory
     @PathSensitive(PathSensitivity.RELATIVE)
     public File getGitDir() {
+        return provideGitDir(workingDir);
+    }
+
+    static File provideGitDir(File workingDir) {
         if (workingDir == null) {
             return null;
         }
-        else {
-            return new File(workingDir, ".git");
-        }
+
+        File gitDir = new File(workingDir, ".git");
+        return gitDir.isDirectory() ? gitDir : null;
     }
 
     public void setWorkingDir(File workingDir) {

src/test/groovy/org/shipkit/changelog/ChangelogPluginTest.groovy

@@ -12,13 +12,6 @@ class ChangelogPluginTest extends Specification {
         project.plugins.apply(ChangelogPlugin)
 
         then:
-        GenerateChangelogTask t = project.tasks.generateChangelog
-        t.gitDir
-
-        when:
-        t.workingDir = null
-
-        then:
-        !t.gitDir
+        project.tasks.generateChangelog
     }
 }

src/test/groovy/org/shipkit/changelog/GenerateChangelogTaskTest.groovy

@@ -0,0 +1,22 @@
+package org.shipkit.changelog
+
+import org.junit.Rule
+import org.junit.rules.TemporaryFolder
+import spock.lang.Specification
+
+import static org.shipkit.changelog.GenerateChangelogTask.provideGitDir
+
+class GenerateChangelogTaskTest extends Specification {
+
+    @Rule TemporaryFolder tmp = new TemporaryFolder()
+
+    def "getGitDir is safe"() {
+        expect:
+        provideGitDir(null) == null
+        provideGitDir(new File("missing")) == null
+
+        and:
+        def gitDir = tmp.newFolder(".git")
+        provideGitDir(gitDir.parentFile) == gitDir
+    }
+}