allow views of undefined columns

src/mol-data/db/column.ts

@@ -364,7 +364,7 @@ function isIdentity(map: ArrayLike<number>, rowCount: number) {
 }
 
 function columnView<T>(c: Column<T>, map: ArrayLike<number>, checkIdentity: boolean): Column<T> {
-    if (!c.isDefined || c.rowCount === 0) return c;
+    if (c.rowCount === 0) return c;
     if (checkIdentity && isIdentity(map, c.rowCount)) return c;
     if (!!c.__array && typeof c.value(0) === typeof c.__array[0]) return arrayView(c, map);
     return viewFull(c, map);