Fix `placement` system crashing ...

... due to block definitions not being loaded yet
(noticeable on web builds)

common/src/block.rs

@@ -41,7 +41,10 @@ pub struct BlockMap(HashMap<BlockPos, Entity>);
 #[derive(SystemParam)]
 pub struct Blocks<'w, 's> {
     map: Res<'w, BlockMap>,
-    definition_collection: Res<'w, BlockDefinitionCollection>,
+    // NOTE: `Option` is used because systems using this parameter might run
+    //       before block definitions are loaded, since they might not have
+    //       a concept of when gameplay is started, for example.
+    definition_collection: Option<Res<'w, BlockDefinitionCollection>>,
     _definition_assets: Res<'w, Assets<BlockDefinition>>,
     blocks: Query<'w, 's, &'static Block>,
     commands: Commands<'w, 's>,
@@ -65,7 +68,11 @@ impl Blocks<'_, '_> {
         pos: BlockPos,
         id: Identifier<Block>,
     ) -> Result<EntityCommands, SpawnBlockError> {
-        if self.definition_collection.get(&id).is_none() {
+        let definitions = self
+            .definition_collection
+            .as_deref()
+            .expect("attempted to spawn block before definitions are loaded");
+        if definitions.get(&id).is_none() {
             return Err(SpawnBlockError::UnknownBlockId(id));
         }
         if let Some(existing) = self.get(pos) {