fix(customwmsbasemaoeditor.tsx): fix error when clearing layers in un…

…defined wms options object

When config.wms is undefined the clearing of the layers array threw an error. This is fixed by
checking if the context.options.config.wms object is undefined or not.

src/editor/CustomWMSBasemapEditor.tsx

@@ -77,7 +77,9 @@ export const CustomWMSBasemapEditor = ({ item, value, onChange, context }: Props
       <Input value={url} aria-label="URL input"
           onChange={e => {
             setURL(e.currentTarget.value);
-            (context.options.config.wms.layers as string[]).splice(-1);
+            if (context.options.config.wms) {
+              (context.options.config.wms.layers as string[]).splice(-1);
+            }
             setOptions([]);
             setSelection([]);
             }}></Input>