Try and fix the AppVeyor issue.

Author: daniel-raffler

src/org/sosy_lab/java_smt/test/SolverThreadLocalityTest.java

@@ -50,8 +50,10 @@ public void initProblemGenerator() {
 
   @After
   public void releaseThreads() {
-    // All threads should have terminated by now as we always wait in the test cases
-    executor.shutdownNow();
+    // All threads should have terminated by now as we always wait in the test
+    if (executor != null) {
+      executor.shutdownNow();
+    }
   }
 
   @Test