Cannot invoke "io.swagger.v3.oas.models.media.Schema.getName()" because "subtypeModel" is null

[srgva]

Describe the bug

When updating from 2.8.14 to 3.0.1, there is a new exception in the logs after the application is started and swagger UI is opened in the browser:

o.s.c.utils.SpringDocAnnotationsUtils : Graceful exception occurred

java.lang.NullPointerException: Cannot invoke "io.swagger.v3.oas.models.media.Schema.getName()" because "subtypeModel" is null
at io.swagger.v3.core.jackson.ModelResolver.resolveSubtypes(ModelResolver.java:2123) ~[swagger-core-jakarta-2.2.41.jar:2.2.41]
at io.swagger.v3.core.jackson.ModelResolver.resolve(ModelResolver.java:990) ~[swagger-core-jakarta-2.2.41.jar:2.2.41]
at org.springdoc.core.converters.AdditionalModelsConverter.resolve(AdditionalModelsConverter.java:176) ~[springdoc-openapi-starter-common-3.0.1.jar:3.0.1]
at org.springdoc.core.converters.FileSupportConverter.resolve(FileSupportConverter.java:72) ~[springdoc-openapi-starter-common-3.0.1.jar:3.0.1]
at org.springdoc.core.converters.ResponseSupportConverter.resolve(ResponseSupportConverter.java:87) ~[springdoc-openapi-starter-common-3.0.1.jar:3.0.1]
at org.springdoc.core.converters.SchemaPropertyDeprecatingConverter.resolve(SchemaPropertyDeprecatingConverter.java:95) ~[springdoc-openapi-starter-common-3.0.1.jar:3.0.1]
at org.springdoc.core.converters.PolymorphicModelConverter.resolve(PolymorphicModelConverter.java:148) ~[springdoc-openapi-starter-common-3.0.1.jar:3.0.1]
at org.springdoc.core.customizers.KotlinDeprecatedPropertyCustomizer.resolve(KotlinDeprecatedPropertyCustomizer.kt:55) ~[springdoc-openapi-starter-common-3.0.1.jar:3.0.1]

To Reproduce
Steps to reproduce the behavior:

• What version of spring-boot you are using? 4.0.1
• Kotlin 2.3.0
• What modules and versions of springdoc-openapi are you using? 3.0.1
• What is the actual and the expected result using OpenAPI Description (yml or json)?
• Provide with a sample code (HelloController) or Test that reproduces the problem

Expected behavior

No warning should show up.

Screenshots
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Additional context
Add any other context about the problem here.

[Mattias-Sehlstedt]

Hi, I would suggest sharing a minimal controller that reproduces the problem so that it would be easy for someone investigating this issue to verify that the issue has been fixed.