more graceful handling of slice cancellation

src/kiri/function.js

@@ -153,6 +153,7 @@ function prepareSlices(callback, scale = 1, offset = 0) {
                 show.progress(0);
                 client.restart();
                 event.emit('slice.error', error);
+                setTimeout(view.set_arrange, 1);
             }
             if (errored) {
                 // terminate slicing

src/kiri/widget.js

@@ -812,6 +812,7 @@ class Widget {
      */
     render(stack) {
         const mark = Date.now();
+        if (this.slices)
         this.slices.forEach(slice => {
             if (slice.layers) {
                 stack.add(slice.layers);