Fixes for constraint selection.

- Don't error when constraints are cleared while selection is active.
- Don't show "latest" highlight when it is the current selection.

index.html

@@ -381,7 +381,8 @@ <h2>Composite constraints (experimental)</h2>
                 <div>
                   <span class="description">
                     Select a composite constraint by clicking on its chip to
-                    add constraints to it.
+                    add constraints to it. Panels which allow constraints to be
+                    added to the composite will be highlighted in yellow.
                   </span>
                   <button type="button" name="add-or" title="Add 'Or'">
                     + Or

js/constraint_displays.js

@@ -59,7 +59,7 @@ class BaseConstraintDisplayItem extends DisplayItem {
   }
 
   removeItem(item) {
-    item.parentNode.removeChild(item);
+    item.parentNode?.removeChild(item);
   }
 
   _removeCircleFromPath(p0, p1) {
@@ -285,7 +285,7 @@ ConstraintDisplays.Jigsaw = class Jigsaw extends BaseConstraintDisplayItem {
   removeItem(item) {
     if (this._regionElems.has(item)) {
       this._colorPicker.removeItem(item);
-      item.parentNode.removeChild(item);
+      item.parentNode?.removeChild(item);
       this._regionElems.delete(item);
       this._updateMissingRegion();
     }
@@ -417,7 +417,7 @@ ConstraintDisplays.CustomBinary = class CustomBinary extends ConstraintDisplays.
 
   removeItem(item) {
     if (this._colorPicker.removeItem(item)) {
-      item.parentNode.removeChild(item);
+      item.parentNode?.removeChild(item);
     }
   }
 
@@ -606,7 +606,7 @@ ConstraintDisplays.ShadedRegion = class ShadedRegion extends BaseConstraintDispl
 
   removeItem(item) {
     if (this._cellColors.removeItem(item)) {
-      item.parentNode.removeChild(item);
+      item.parentNode?.removeChild(item);
     }
   }
 
@@ -734,7 +734,7 @@ ConstraintDisplays.CountingCircles = class CountingCircles extends BaseConstrain
 
   removeItem(item) {
     if (this._circleColors.removeItem(item)) {
-      item.parentNode.removeChild(item);
+      item.parentNode?.removeChild(item);
     }
   }
 
@@ -945,7 +945,7 @@ ConstraintDisplays.BorderedRegion = class BorderedRegion extends BaseConstraintD
 
   removeItem(item) {
     if (this._colorPicker.removeItem(item)) {
-      item.parentNode.removeChild(item);
+      item.parentNode?.removeChild(item);
     }
   }
 

js/render_page.js

@@ -1527,7 +1527,9 @@ class ConstraintSelector {
   }
 
   updateLatest(constraint, chip) {
-    this._latestHighlighter.setConstraint(constraint, chip);
+    if (constraint !== this._selectionHighlighter.currentConstraint()) {
+      this._latestHighlighter.setConstraint(constraint, chip);
+    }
   }
 
   select(constraint, chip, collection) {