Guard against `None` chart in `ArrowEditor.remove()`

Add null check for `linked.chart` before calling
`.plotItem.removeItem()` to prevent `AttributeError` when chart
is `None`.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/ui/_editors.py

@@ -169,7 +169,10 @@ class ArrowEditor(Struct):
             f'{arrow!r}\n'
         )
         for linked in self.godw.iter_linked():
-            linked.chart.plotItem.removeItem(arrow)
+            if not (chart := linked.chart):
+                continue
+
+            chart.plotItem.removeItem(arrow)
         try:
             arrows.remove(arrow)
         except ValueError: