From 0b6cad97355144d0ad30e4bce699fd1be6e2c512 Mon Sep 17 00:00:00 2001
From: andriyDev <andriydzikh@gmail.com>
Date: Tue, 14 Oct 2025 15:54:48 -0700
Subject: [PATCH] Allow DiagnosticsBuffer to be applied even if
 DiagnosticsStore is missing.

---
 crates/bevy_diagnostic/src/diagnostic.rs | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/crates/bevy_diagnostic/src/diagnostic.rs b/crates/bevy_diagnostic/src/diagnostic.rs
index 937da5b258502..17943ae60d308 100644
--- a/crates/bevy_diagnostic/src/diagnostic.rs
+++ b/crates/bevy_diagnostic/src/diagnostic.rs
@@ -381,7 +381,17 @@ impl SystemBuffer for DiagnosticsBuffer {
         _system_meta: &bevy_ecs::system::SystemMeta,
         world: &mut bevy_ecs::world::World,
     ) {
-        let mut diagnostics = world.resource_mut::<DiagnosticsStore>();
+        let Some(mut diagnostics) = world.get_resource_mut::<DiagnosticsStore>() else {
+            // `SystemBuffer::apply` is called even if the system never runs. If a user uses
+            // `If<Diagnostics>`, this buffer will be applied even if we are missing
+            // `DiagnosticsStore`. So be permissive to allow these cases. See
+            // https://github.com/bevyengine/bevy/issues/21549 for more.
+
+            // Clear the buffer since we have nowhere to put those metrics and we don't want them to
+            // grow without bound.
+            self.0.clear();
+            return;
+        };
         for (path, measurement) in self.0.drain() {
             if let Some(diagnostic) = diagnostics.get_mut(&path) {
                 diagnostic.add_measurement(measurement);