Fix crash when removing a link mark with no active mark present

The link toolbar's onRemove handler called editor.removeMark()/
removeMarkAtRange() with the result of an unguarded Array.find(),
which is undefined whenever the selection has no active link mark.
Slate's Mark.create() silently defaults an undefined argument to {}
and passes it to Mark.fromJSON(), which throws "Mark.fromJS()
requires a type string." This is reached whenever a user opens the
Link popover and confirms an empty URL field without ever having
applied a link (Enter or the Add button triggers onConfirm, which
delegates to onRemove when the field is blank).

Guard onRemove the same way the neighboring removeMarksOfTypeInRange
helper already does: no-op when there's no active mark to remove.

Author: claude

app/src/components/composer-editor/toolbar-component-factories.tsx

@@ -201,6 +201,13 @@ export function BuildMarkButtonWithValuePicker(config) {
       e.preventDefault();
       const { value, editor } = this.props;
       const active = safeActiveMarks(value).find((m) => m.type === config.type);
+      if (!active) {
+        // Nothing to remove — e.g. the user confirmed an empty URL field
+        // without ever having applied a mark. Calling into Slate with an
+        // undefined mark throws inside Mark.fromJSON.
+        editor.focus();
+        return;
+      }
       if (value.selection.isCollapsed) {
         const anchorNode = value.document.getNode(value.selection.anchor.key);
         const expanded = value.selection.moveToRangeOfNode(anchorNode);