fix tryGetCellData data check

CHANGELOG.md

@@ -6,7 +6,7 @@ Note that since we don't clearly distinguish between a public and private interf
 
 ## [Unreleased]
 
-- Fix return type of `State.tryGetCellData`
+- Fix `State.tryGetCellData` (return type & data check)
 - Don't change camera.target unless flyMode or pointerLock are enabled
 - Handle empty CIF files
 - Snapshot improvements:

src/mol-state/state.ts

@@ -71,7 +71,7 @@ class State {
 
     tryGetCellData = <T extends StateObject>(ref: StateTransform.Ref) => {
         const ret = this.cells.get(ref)?.obj?.data;
-        if (!ref) throw new Error(`Cell '${ref}' data undefined.`);
+        if (ret === undefined) throw new Error(`Cell '${ref}' data undefined.`);
         return ret as T extends StateObject<infer D> ? D : never;
     };