Stack layer: fix a UI hang-up on reloading wrong mirror IDs

This was caused by endlessly switching mirrors to a non-existent mirror
ID.

Author: tomka

CHANGELOG.md

@@ -294,6 +294,9 @@ Miscellaneous:
 - Block image datasets like N5 or Neuroglancer now don't lead to errors anymore
   when their size isn't anymore a multiple of the block size.
 
+- Restoring saved stack layer settings: a UI hang-up with fixed, caused by
+  endlessly switching mirrors to a non-existent mirror ID.
+
 ## Maintenance updates
 
 - Node distance measurements: computation of straight line distance has been

django/applications/catmaid/static/js/layers/stack-layer.js

@@ -648,6 +648,11 @@
    */
   StackLayer.prototype.switchToMirror = function (mirrorId, force) {
     if (mirrorId === this.mirrorId && !force) return;
+    // If the referenced mirror doesn't exist and is no custom mirror, then
+    // don't attempt to switch.
+    if (this.stack.mirrors[mirrorId] === undefined && this.getCustomMirrorIds().indexOf(mirrorId) == -1) {
+      return;
+    }
     var newStackLayer = this.constructCopy({mirrorId: mirrorId});
     var layerKey = this.stackViewer.getLayerKey(this);
     this.stackViewer.replaceStackLayer(layerKey, newStackLayer);