Quick Answer

Incorrect use of open on local variables

Understanding the Issue

The open modifier allows classes and members to be overridden in subclasses, but it's not applicable to local variables, which can't be subclassed or overridden.

The Problem

This code demonstrates the issue:

Kotlin Error 
// Problem: Invalid open modifier
fun example() {
    open val local = 1 // Error
}

The Solution

Here's the corrected code:

Kotlin Fixed 
// Solution: Remove open from local
fun example() {
    val local = 1 // Correct
}

// Proper use of open:
open class Base {
    open val prop = 1 // Allowed
}

Key Takeaways

Only use open on class members that should be overridable in subclasses.