Fix handling of NestedServletException with no root cause

Fixes gh-22169

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/support/ErrorPageFilter.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -137,7 +137,10 @@ public class ErrorPageFilter implements Filter, ErrorPageRegistry {
 		catch (Throwable ex) {
 			Throwable exceptionToHandle = ex;
 			if (ex instanceof NestedServletException) {
-				exceptionToHandle = ((NestedServletException) ex).getRootCause();
+				Throwable rootCause = ((NestedServletException) ex).getRootCause();
+				if (rootCause != null) {
+					exceptionToHandle = rootCause;
+				}
 			}
 			handleException(request, response, wrapped, exceptionToHandle);
 			response.flushBuffer();

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/servlet/support/ErrorPageFilterTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -42,6 +42,7 @@ import org.springframework.mock.web.MockFilterConfig;
 import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.mock.web.MockHttpServletResponse;
 import org.springframework.mock.web.MockRequestDispatcher;
+import org.springframework.web.bind.MissingServletRequestParameterException;
 import org.springframework.web.context.request.async.DeferredResult;
 import org.springframework.web.context.request.async.StandardServletAsyncWebRequest;
 import org.springframework.web.context.request.async.WebAsyncManager;
@@ -388,6 +389,30 @@ public class ErrorPageFilterTests {
 		assertThat(this.response.getForwardedUrl()).isEqualTo("/500");
 	}
 
+	@Test
+	public void nestedServletExceptionWithNoCause() throws Exception {
+		this.filter.addErrorPages(new ErrorPage(MissingServletRequestParameterException.class, "/500"));
+		this.chain = new TestFilterChain((request, response, chain) -> {
+			chain.call();
+			throw new MissingServletRequestParameterException("test", "string");
+		});
+		this.filter.doFilter(this.request, this.response, this.chain);
+		assertThat(((HttpServletResponseWrapper) this.chain.getResponse()).getStatus()).isEqualTo(500);
+		assertThat(this.request.getAttribute(RequestDispatcher.ERROR_STATUS_CODE)).isEqualTo(500);
+		assertThat(this.request.getAttribute(RequestDispatcher.ERROR_MESSAGE))
+				.isEqualTo("Required string parameter 'test' is not present");
+		Map<String, Object> requestAttributes = getAttributesForDispatch("/500");
+		assertThat(requestAttributes.get(RequestDispatcher.ERROR_EXCEPTION_TYPE))
+				.isEqualTo(MissingServletRequestParameterException.class);
+		assertThat(requestAttributes.get(RequestDispatcher.ERROR_EXCEPTION))
+				.isInstanceOf(MissingServletRequestParameterException.class);
+		assertThat(this.request.getAttribute(RequestDispatcher.ERROR_EXCEPTION_TYPE)).isNull();
+		assertThat(this.request.getAttribute(RequestDispatcher.ERROR_EXCEPTION)).isNull();
+		assertThat(this.request.getAttribute(RequestDispatcher.ERROR_REQUEST_URI)).isEqualTo("/test/path");
+		assertThat(this.response.isCommitted()).isTrue();
+		assertThat(this.response.getForwardedUrl()).isEqualTo("/500");
+	}
+
 	@Test
 	public void whenErrorIsSentAndWriterIsFlushedErrorIsSentToTheClient() throws Exception {
 		this.chain = new TestFilterChain((request, response, chain) -> {