catch empty trails log when trying to generate a vector trails costume

HISTORY.md

@@ -14,6 +14,7 @@
 * threads: new "pen trails (SVG)" reporter (experimental, hidden in dev)
 * objects, threads: adjusted rotation center of SVG-pen-trails
 * objects: added aspect racio governance and generator tags to trails SVGs
+* threads: catch empty trails log when trying to generate a vector trails costume
 
 ## v5.3.8:
 * **Notable Change:**

src/threads.js

@@ -5118,6 +5118,11 @@ Process.prototype.reportPentrailsAsSVG = function () { // +++
 
     if (!this.context.accumulator) {
         stage = this.homeContext.receiver.parentThatIsA(StageMorph);
+        if (!stage.trailsLog.length) {
+            throw new Error (localize(
+                'there are currently no\nvectorizable pen trail segments'
+            ));
+        }
         svg = stage.trailsLogAsSVG();
         this.context.accumulator = {
             img : new Image(),