test to reproduce the issue 3787

Author: kliushnichenko

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

@@ -0,0 +1,23 @@
+package io.jooby.i3787;
+
+import io.jooby.Jooby;
+import io.jooby.problem.HttpProblem;
+
+class Application extends Jooby {
+  {
+    get("/throw", ctx -> {
+      throw new CustomException();
+    });
+
+    error(CustomException.class, (ctx, throwable, statusCode) -> {
+      var problem = HttpProblem.badRequest("A Client Error — Obviously");
+      ctx.getRouter().getErrorHandler().apply(ctx, problem, statusCode);
+    });
+  }
+
+  public static void main(String[] args) {
+    runApp(args, Application::new);
+  }
+
+  static class CustomException extends RuntimeException {}
+}

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

@@ -0,0 +1,20 @@
+package io.jooby.i3787;
+
+import io.jooby.StatusCode;
+import io.jooby.test.MockContext;
+import io.jooby.test.MockRouter;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue3787 {
+
+  private final MockRouter router = new MockRouter(new Application()).setFullExecution(true);
+
+  @Test
+  void test() {
+    var ctx = new MockContext();
+    router.tryError(new Application.CustomException(), ctx); // throws java.lang.NullPointerException
+    assertEquals(StatusCode.BAD_REQUEST, ctx.getResponseCode());
+  }
+}