FIX: ensures rects is present before using it (#11930)

I don't have a clear reproduction ATM, but I imagine that in fast tests element can get destroyed before we get to use it.

app/assets/javascripts/select-kit/addon/components/select-kit.js

@@ -883,7 +883,10 @@ export default Component.extend(
                   wrapper.style.height = `${height}px`;
                   if (placementStrategy === "fixed") {
                     const rects = this.element.getClientRects()[0];
+
+                    if (rects) {
                       const bodyRects = body && body.getClientRects()[0];
+
                       wrapper.style.position = "fixed";
                       wrapper.style.left = `${rects.left}px`;
                       if (topPlacement && bodyRects) {
@@ -896,6 +899,7 @@ export default Component.extend(
                       }
                     }
                   }
+                }
               },
             },
           ],