fix NPE

Author: kliushnichenko

modules/jooby-test/src/main/java/io/jooby/test/MockRouter.java

@@ -472,6 +472,8 @@ public void tryError(Throwable cause, Consumer<MockResponse> consumer) {
    */
   public void tryError(Throwable cause, Context ctx) {
     var app = supplier.get();
+    MockContext findContext = ctx instanceof MockContext ? (MockContext) ctx : newContext();
+    findContext.setRouter(app);
     var handler = app.getErrorHandler();
     handler.apply(ctx, cause, app.errorCode(cause));
   }

tests/src/test/java/io/jooby/i3787/Issue3787.java

@@ -12,9 +12,9 @@ public class Issue3787 {
   private final MockRouter router = new MockRouter(new Application()).setFullExecution(true);
 
   @Test
-  void test() {
+  void issue3787() {
     var ctx = new MockContext();
-    router.tryError(new Application.CustomException(), ctx); // throws java.lang.NullPointerException
+    router.tryError(new Application.CustomException(), ctx);
     assertEquals(StatusCode.BAD_REQUEST, ctx.getResponseCode());
   }
 }