[Bug] NPE in KotlinDeprecatedPropertyCustomizer - resolvedSchema is null

[youming-lin]

Describe the bug

Stack trace:

java.lang.NullPointerException: Cannot invoke "io.swagger.v3.oas.models.media.Schema.get$ref()" because "resolvedSchema" is null
	at org.springdoc.core.customizers.KotlinDeprecatedPropertyCustomizer.resolve(KotlinDeprecatedPropertyCustomizer.kt:67) ~[springdoc-openapi-starter-common-2.8.13.jar:2.8.13]
	at org.springdoc.core.converters.KotlinInlineClassUnwrappingConverter.resolve(KotlinInlineClassUnwrappingConverter.kt:42) ~[springdoc-openapi-starter-common-2.8.13.jar:2.8.13]
	at io.swagger.v3.core.converter.ModelConverterContextImpl.resolve(ModelConverterContextImpl.java:97) ~[swagger-core-jakarta-2.2.36.jar:2.2.36]
	at io.swagger.v3.core.jackson.ModelResolver.resolve(ModelResolver.java:824) ~[swagger-core-jakarta-2.2.36.jar:2.2.36]
	at org.springdoc.core.converters.AdditionalModelsConverter.resolve(AdditionalModelsConverter.java:176) ~[springdoc-openapi-starter-common-2.8.13.jar:2.8.13]
	at org.springdoc.core.converters.FileSupportConverter.resolve(FileSupportConverter.java:72) ~[springdoc-openapi-starter-common-2.8.13.jar:2.8.13]
	at org.springdoc.core.converters.ResponseSupportConverter.resolve(ResponseSupportConverter.java:87) ~[springdoc-openapi-starter-common-2.8.13.jar:2.8.13]
	at org.springdoc.core.converters.SchemaPropertyDeprecatingConverter.resolve(SchemaPropertyDeprecatingConverter.java:95) ~[springdoc-openapi-starter-common-2.8.13.jar:2.8.13]
<rest omitted>

I'm upgrading app to Spring Boot 3, which necessitated Springdoc update. I'm using v2.8.13.

The bug is caused when the following conditions are met:

1. Usage of Kotlin
2. Usage of @JsonUnwrapped
3. Usage of @Deprecated in the nested object that is being unwrapped

To Reproduce
Steps to reproduce the behavior:

• What version of spring-boot you are using?
  v3.5.7
• What modules and versions of springdoc-openapi are you using?
  springdoc-openapi-starter-webmvc-ui, v2.8.13
• What is the actual and the expected result using OpenAPI Description (yml or json)?
  Actual: affected type is missing in yaml
  Expected: the type is correctly generated and included in yml
• Provide with a sample code (HelloController) or Test that reproduces the problem

Sample code to reproduce error:

package com.example.demo

import com.fasterxml.jackson.annotation.JsonUnwrapped
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController

@RestController
@RequestMapping("/test")
@SpringBootApplication
class DemoApp {
    @GetMapping
    fun getTestData(): Foo = Foo(bar = Bar(baz = "test"))
}

data class Foo(
    @field:JsonUnwrapped
    val bar: Bar,
)

data class Bar(
    @Deprecated("This is deprecated")
    val baz: String,
)

fun main(args: Array<String>) {
    runApplication<DemoApp>(*args)
}

pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://maven.apache.org/POM/4.0.0"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.5.7</version>
    </parent>
    <groupId>com.example</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>KotlinDeprecatedPropertyCustomizer-bug</name>
    <description>Example showing bug in KotlinDeprecatedPropertyCustomizer</description>
    <properties>
        <java.version>21</java.version>
        <kotlin.version>2.0.20</kotlin.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-reflect</artifactId>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-test-junit5</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springdoc</groupId>
            <artifactId>springdoc-openapi-starter-webmvc-ui</artifactId>
            <version>2.8.13</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-annotations</artifactId>
        </dependency>
    </dependencies>

    <build>
        <sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory>
        <testSourceDirectory>${project.basedir}/src/test/kotlin</testSourceDirectory>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
            <plugin>
                <groupId>org.jetbrains.kotlin</groupId>
                <artifactId>kotlin-maven-plugin</artifactId>
                <configuration>
                    <args>
                        <arg>-Xjsr305=strict</arg>
                    </args>
                    <compilerPlugins>
                        <plugin>spring</plugin>
                    </compilerPlugins>
                </configuration>
                <dependencies>
                    <dependency>
                        <groupId>org.jetbrains.kotlin</groupId>
                        <artifactId>kotlin-maven-allopen</artifactId>
                        <version>${kotlin.version}</version>
                    </dependency>
                </dependencies>
            </plugin>
        </plugins>
    </build>

</project>

After server starts, hit http://localhost:8080/swagger-ui/index.html. Observe exception in logs.

Expected behavior

• A clear and concise description of what you expected to happen.
  No exception, and type is included in yaml.
• What is the expected result using OpenAPI Description (yml or json)?
  Yaml should include the response type; currently it's missing.

Screenshots
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Additional context
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