Mutators throw NullPointerException when used with MockMvc setup

When MockMvc is used to set up a WebTestClient, applying a mutator such as csrf() fails with a NullPointerException, but would ideally fail with a more meaningful error such as IllegalStateException to indicate it is not supported. The following Spring Boot tests illustrate the problem:

@WebMvcTest
public class WebTestClientTests {
	@Autowired
	private MockMvc mockMvc;

	@Test
	public void mockUserDetailsWhenMockMvcThenThrowsIllegalStateException() {
		UserDetails user = new User("user", "password", Collections.emptySet());
		assertThatIllegalStateException()
				.isThrownBy(() -> MockMvcWebTestClient.bindTo(mockMvc).apply(mockUser(user)).build());
	}

	@Test
	public void mockUserWhenMockMvcThenThrowsIllegalStateException() {
		assertThatIllegalStateException()
				.isThrownBy(() -> MockMvcWebTestClient.bindTo(mockMvc).apply(mockUser("user")).build());
	}

	@Test
	public void csrfWhenMockMvcThenThrowsIllegalStateException() {
		assertThatIllegalStateException()
				.isThrownBy(() -> MockMvcWebTestClient.bindTo(mockMvc).apply(csrf()).build());
	}

	@Test
	public void mockJwtWhenMockMvcThenThrowsIllegalStateException() {
		assertThatIllegalStateException()
				.isThrownBy(() -> MockMvcWebTestClient.bindTo(mockMvc).apply(mockJwt()).build());
	}

	@Test
	public void mockOpaqueTokenWhenMockMvcThenThrowsIllegalStateException() {
		assertThatIllegalStateException()
				.isThrownBy(() -> MockMvcWebTestClient.bindTo(mockMvc).apply(mockOpaqueToken()).build());
	}

	@Test
	public void mockOAuth2ClientWhenMockMvcThenThrowsIllegalStateException() {
		assertThatIllegalStateException()
				.isThrownBy(() -> MockMvcWebTestClient.bindTo(mockMvc).apply(mockOAuth2Client("test")).build());
	}

	@Test
	public void mockOAuth2LoginWhenMockMvcThenThrowsIllegalStateException() {
		assertThatIllegalStateException()
				.isThrownBy(() -> MockMvcWebTestClient.bindTo(mockMvc).apply(mockOAuth2Login()).build());
	}

	@Test
	public void mockOidcLoginWhenMockMvcThenThrowsIllegalStateException() {
		assertThatIllegalStateException()
				.isThrownBy(() -> MockMvcWebTestClient.bindTo(mockMvc).apply(mockOidcLogin()).build());
	}
}

Related gh-10841 gh-10900

Start with MockMvcWebTestClient.bindTo(mockMvc) and the mutator calls shown in the Spring Boot tests. Add coverage for the listed mutators and verify that each unsupported MockMvc setup fails with IllegalStateException rather than NullPointerException.