[Kotlin] generated code: <field>AsByteBuffer throws NullPointerException when field is null

[souma987]

When using Kotlin code generated by flatc, calling <field>AsByteBuffer on a nullable string field throws a NullPointerException. The Java generated code for the same schema correctly returns null.

Steps to Reproduce:

Schema (nullable_string.fbs):

table NullableString {
  value:string;
}
root_type NullableString;

Kotlin code:

package org.example

import com.google.flatbuffers.FlatBufferBuilder
import java.nio.ByteBuffer

fun serialize(): ByteBuffer {
    val builder = FlatBufferBuilder()
    val offset = NullableStringKotlin.createNullableString(builder, 0)
    builder.finish(offset)
    return builder.dataBuffer()
}

fun main() {
    val javaRoot = NullableStringJava.getRootAsNullableString(serialize())
    println(javaRoot.value())              // prints "null"
    println(javaRoot.valueAsByteBuffer())  // prints "null"

    val kotlinRoot = NullableStringKotlin.getRootAsNullableString(serialize())
    println(kotlinRoot.value)              // prints "null"
    println(kotlinRoot.valueAsByteBuffer)  // accessing this property throws NullPointerException
}

Output:

null
null
null
Exception in thread "main" java.lang.NullPointerException: __vector_as_bytebuffer(...) must not be null
	at org.example.NullableStringKotlin.getValueAsByteBuffer(NullableStringKotlin.kt:28)
	at org.example.MainKt.main(Main.kt:20)
	at org.example.MainKt.main(Main.kt)

Expected Behavior:
Kotlin generated code should behave the same as Java generated code, returning null when the field is not present.

Environment:

• flatc version: 25.2.10
• FlatBuffers library: com.google.flatbuffers:flatbuffers-java:25.2.10
• Kotlin version: 2.2.0
• Language: Kotlin + Java interop
• JVM: Microsoft OpenJDK 17.0.15
• OS: Windows 11

Possible Fix
Change the generated Kotlin code to return a nullable type.
Instead of:

val valueAsByteBuffer : ByteBuffer get() = __vector_as_bytebuffer(4, 1)

Change to:

val valueAsByteBuffer : ByteBuffer? get() = __vector_as_bytebuffer(4, 1)

This removes the null check that Kotlin automatically performs.

[jtdavis777]

hey @souma987 - thank you for the detailed issue report! Would you be up for creating a PR to fix this? Possibly with a new test? :D

[souma987]

Thanks, I would be happy to! Please allow me a week or so to complete this.