Protecting selectlayer against edge cases.

src/editor/components/selectlayer.tsx

@@ -60,7 +60,11 @@ function SelectLayer({
     };
 
     const handlePointerUp = (e: React.PointerEvent<HTMLDivElement>) => {
-        if (!isEnabled) {
+        if (
+            !isEnabled ||
+            selectionStart == undefined ||
+            selectionEnd == undefined
+        ) {
             return;
         }
         e.preventDefault();