[CSharp] Crash on 'allOf's that contain a '$ref' to 'allOf's

Description

If we have a $ref in an allOf that points to an allOf of $refs codegen crashes with a NullPointerException.

Swagger-codegen version

CLI: 3.0.10

Swagger declaration file content or url

Here is a cut-down example file that seems to reproduce our issue. Our example is obviously more complex as the referenced schemas are used more than once.

https://gist.github.com/thomashardwick/13a8d5516c7e0718e212f6b396fd39e5

Command line used for generation

java -jar .\swagger-codegen-cli-3.0.10.jar generate -l csharp -i .\test.yaml

Steps to reproduce

Run command. Get output:

10:50:11.571 [Thread-0] DEBUG io.swagger.codegen.v3.config.CodegenConfigurator - getClientOptInput - parsed inputSpecURL .\test.yaml
10:50:11.579 [Thread-0] INFO io.swagger.codegen.v3.generators.dotnet.CSharpClientCodegen - Generating code for .NET Framework v4.5
10:50:11.663 [Thread-0] INFO io.swagger.codegen.v3.generators.dotnet.CSharpClientCodegen - adding parent variable Name
Exception in thread "Thread-0" java.lang.RuntimeException: Could not process model 'GroupUserItem'.Please make sure that your schema is correct!
        at io.swagger.codegen.v3.DefaultGenerator.generateModels(DefaultGenerator.java:392)
        at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:773)
        at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:342)
        at java.base/java.lang.Thread.run(Thread.java:835)
Caused by: java.lang.NullPointerException
        at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromModel(DefaultCodegenConfig.java:1336)
        at io.swagger.codegen.v3.generators.dotnet.CSharpClientCodegen.fromModel(CSharpClientCodegen.java:515)
        at io.swagger.codegen.v3.generators.DefaultCodegenConfig.fromModel(DefaultCodegenConfig.java:1239)
        at io.swagger.codegen.v3.generators.dotnet.CSharpClientCodegen.fromModel(CSharpClientCodegen.java:519)
        at io.swagger.codegen.v3.DefaultGenerator.processModels(DefaultGenerator.java:1005)
        at io.swagger.codegen.v3.DefaultGenerator.generateModels(DefaultGenerator.java:363)
        ... 3 more

Start with the reduced schema in the linked gist and reproduce it using the shown swagger-codegen CLI command. Then inspect DefaultCodegenConfig.fromModel and CSharpClientCodegen.fromModel from the stack trace; done means the C# generator handles the nested allOf/$ref case without a NullPointerException and has a regression test.