Loading and using a font after startup can lead to panic.

[BorisBoutillier]

If you load asynchronous a font through the assert_server and directly use the font_handle, panic can happen if this is done in a regular system, after startup.
I have modified the text.rs to show the panic. Modification is that the commands and asset loading are not done at startup but on a specific frame number.

Here is the modified example:

use bevy::{
    diagnostic::{Diagnostics, FrameTimeDiagnosticsPlugin},
    prelude::*,
};

/// This example illustrates how to create text and update it in a system. It displays the current FPS in the upper left hand corner.
fn main() {
    App::build()
        .add_default_plugins()
        .add_plugin(FrameTimeDiagnosticsPlugin::default())
        .add_resource(1u32)
        .add_system(setup.system())
        .add_system(text_update_system.system())
        .run();
}
fn text_update_system(diagnostics: Res<Diagnostics>, mut query: Query<&mut Text>) {
    for mut text in &mut query.iter() {
        if let Some(fps) = diagnostics.get(FrameTimeDiagnosticsPlugin::FPS) {
            if let Some(average) = fps.average() {
                text.value = format!("FPS: {:.2}", average);
            }
        }
    }
}

const FRAME_SPAWN: u32 = 3;

fn setup(mut commands: Commands, count: Res<u32>, asset_server: Res<AssetServer>) {
    if *count <= FRAME_SPAWN {
        println!("FRAME {}", *count);
        if *count == FRAME_SPAWN {
            let font_handle = asset_server.load("assets/fonts/FiraSans-Bold.ttf").unwrap();
            commands
                // 2d camera
                .spawn(UiCameraComponents::default())
                // texture
                .spawn(TextComponents {
                    style: Style {
                        align_self: AlignSelf::FlexEnd,
                        ..Default::default()
                    },
                    text: Text {
                        value: "FPS:".to_string(),
                        font: font_handle,
                        style: TextStyle {
                            font_size: 60.0,
                            color: Color::WHITE,
                        },
                    },
                    ..Default::default()
                });
        }
        commands.insert_resource(*count + 1);
    }
}

Here is the panic message :

thread 'Compute Task Pool (2)' panicked at 'called `Option::unwrap()` on a `None` value', crates/bevy_text/src/font_atlas_set.rs:53:42

[BorisBoutillier]

I think the issue is that some code in bevy_text and bevy_ui is doing fonts.get(font_handle).unwrap(), but because asset loading is asynchronous, the Assets is not yet filled with the associated font.

I'm preparing a PR for that.

[BorisBoutillier]

Note that the same panic will happen by just modifying the original text.rs example and pointing to a non-existent font path.

[BorisBoutillier]

Fixed by #490