slint-ui/slint · error

window_adapter just initialized above

Error message

window_adapter just initialized above

What it means

`window_adapter_ref()` calls `try_window_adapter()` which lazily creates the window adapter, then unwraps `window_adapter.get()` expecting it to be initialized. The `expect` fires if `try_window_adapter()` returned `Ok` without actually installing an adapter into the `OnceCell` — a broken contract between the two calls, or a race/bug in the lazy-init code path. It is an internal invariant violation in the interpreter's component instance.

Solutions

1. Check `top_level_type()` before asking for a window adapter — non-windowed roots may not have one
2. Upgrade to a fixed Slint version; this panic indicates a violated init contract
3. Avoid calling `window_adapter_ref()` on instances you did not `run()`/`show()`; use `try_window_adapter` semantics via the public API instead
4. File a reproducer upstream if a windowed component still triggers it

Example fix

// before
let adapter = instance.window_adapter_ref()?;
// after
if instance.top_level_type() == i_slint_compiler::llr::TopLevelComponentType::Window {
    let adapter = instance.window_adapter_ref()?;
}

if instance.top_level_type() == i_slint_compiler::llr::TopLevelComponentType::Window {
    let adapter = instance.window_adapter_ref()?;
}

When it happens

Trigger: Calling `ComponentInstance::window_adapter_ref()` when `try_window_adapter()` succeeds but leaves the `window_adapter` cell empty — typically after internal refactors of the lazy initialization, or when the component was instantiated without window support and the init path short-circuited incorrectly.

Common situations: Hit by interpreter users calling `window_adapter_ref()` on instances created for non-windowed roots or in headless/testing setups where adapter creation silently skipped installation.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• Cannot get property
• Cannot invoke callback
• Cannot set callback
• Cannot set property
• Global was just valid

AI-assisted analysis of slint-ui/slint@3a7e700487 (2026-09-16). Data as JSON: /api/errors/1910b6d038c0423c. Report an issue: GitHub.

Appendix: source

Thrown at internal/interpreter/component.rs:298

        global: &str,
        name: &str,
        handler: impl Fn(&[Value]) -> Value + 'static,
    ) -> Result<(), ()> {
        public_api::set_global_callback(&self.0, global, name, Box::new(handler))
    }

    pub fn invoke_global(&self, global: &str, name: &str, args: &[Value]) -> Option<Value> {
        public_api::invoke_global(&self.0, global, name, args)
    }

    /// Return a borrowed reference to the window adapter, creating one
    /// through the backend selector if necessary. The returned reference
    /// lives as long as the instance.
    pub fn window_adapter_ref(
        &self,
    ) -> Result<&i_slint_core::window::WindowAdapterRc, i_slint_core::api::PlatformError> {
        self.0.try_window_adapter()?;
        Ok(self.0.window_adapter.get().expect("window_adapter just initialized above"))
    }

    /// Whether the root inherits `Window` or a non-windowed type such as
    /// `SystemTrayIcon`.
    pub fn top_level_type(&self) -> i_slint_compiler::llr::TopLevelComponentType {
        let unit = &self.0.root_sub_component.compilation_unit;
        match self.0.public_component_index {
            Some(idx) => unit.public_components[idx].top_level_type,
            None => i_slint_compiler::llr::TopLevelComponentType::Window,
        }
    }

    /// Definition this instance was created from.
    pub fn definition(&self) -> ComponentDefinitionInner {
        let public_index = self.0.public_component_index.unwrap_or(0.into());
        ComponentDefinitionInner {
            compilation_unit: self.0.root_sub_component.compilation_unit.clone(),
            public_index,

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