[Bug]: A NullPointerException is generated when running bal doc

[nipunayf]

Description

The following NullPointerException message is generated when running the bal doc command after exposing a union type through a public type.

# Description
public type PublicType PrivateType;

type PrivateType string|int;

Generating API Documentation
ballerina: Oh no, something really went wrong. Bad. Sad.

We appreciate it if you can report the code that broke Ballerina in
https://github.com/ballerina-platform/ballerina-lang/issues with the
log you get below and your sample code.

We thank you for helping make us better.

[2022-10-11 13:09:51,987] SEVERE {b7a.log.crash} - null 
java.lang.NullPointerException
        at org.ballerinalang.docgen.Generator.addTypeDefinition(Generator.java:192)
        at org.ballerinalang.docgen.Generator.setModuleFromSyntaxTree(Generator.java:122)
        at org.ballerinalang.docgen.docs.BallerinaDocGenerator.getDocsGenModel(BallerinaDocGenerator.java:484)
        at org.ballerinalang.docgen.docs.BallerinaDocGenerator.generateAPIDocs(BallerinaDocGenerator.java:195)
        at io.ballerina.cli.task.CreateDocsTask.execute(CreateDocsTask.java:60)
        at io.ballerina.cli.TaskExecutor.executeTasks(TaskExecutor.java:40)
        at io.ballerina.cli.cmd.DocCommand.execute(DocCommand.java:185)
        at java.base/java.util.Optional.ifPresent(Optional.java:183)
        at io.ballerina.cli.launcher.Main.main(Main.java:51)

However, the command works if the PrivateType is public or it only consists of one type as, type PrivateType int;

Steps to Reproduce

1. Create a new package and add the following lines in the main.bal

# Description
public type PublicType PrivateType;

type PrivateType string|int;

2. Run the bal doc command on the package.

Affected Version(s)

2201.2.1

OS, DB, other environment details and versions

OS - Ubuntu 22.04.1 LTS

Related area

-> API Docs

Related issue(s) (optional)

No response

Suggested label(s) (optional)

No response

Suggested assignee(s) (optional)

No response