Swagger not rendered when allOf used in schema

https://github.com/swagger-api/swagger-codegen-generators/blob/dff6a3e4c56462a24873fd62e3c38ba61ed7974f/src/main/java/io/swagger/codegen/v3/generators/SchemaHandler.java#L20

I got null pointer execption on combined schema with allOf.
According to the doc
https://swagger.io/docs/specification/data-models/oneof-anyof-allof-not/
allOf, anyOf...etc should be lower case.
Although swagger-apidoc generates an issue when allOf us used with lowercase (first char).

When I change allOf to AllOf, the swagger doc renders.

The exception is the following using lowercase allOf:

  Exception in thread "Thread-1" java.lang.NullPointerException
	  at io.swagger.codegen.v3.generators.SchemaHandler.processArrayItemSchema(SchemaHandler.java:127)
	  at io.swagger.codegen.v3.generators.SchemaHandler.processComposedSchemas(SchemaHandler.java:39)
	  at io.swagger.codegen.v3.ISchemaHandler.readProcessedModels(ISchemaHandler.java:30)
	  at io.swagger.codegen.v3.DefaultGenerator.generateModels(DefaultGenerator.java:391)
	  at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:779)
	  at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:360)
	  at java.lang.Thread.run(Thread.java:748)

Tested on version an older version 1.0.20, but the issue based on the source code seems to be present in the master branch too.

Example json schema:

    "ConfigUpdateRequestDto": {
      "type": "array",
      "items": {
        "AllOf": [
          {
            "$ref": "#/components/schemas/Config"
          },
          {
            "type": "object",
            "properties": {
              "apply": {
                "type": "boolean"
              }
            }
          }
        ]
      }
    },

Start with src/main/java/io/swagger/codegen/v3/generators/SchemaHandler.java, especially processArrayItemSchema at line 127 and processComposedSchemas, then reproduce the NullPointerException using the provided ConfigUpdateRequestDto schema. Done means a schema using lowercase allOf is processed without the exception and the Swagger documentation renders correctly.