fix: Check if saved separator is a string (#944)

Author: kojiishi

src/document_applier.ts

@@ -89,7 +89,9 @@ export class DocumentApplier {
             separator: this.separator,
           },
           items => {
-            this.separator = items.separator;
+            if (typeof items.separator === 'string') {
+              this.separator = items.separator;
+            }
             resolve();
           }
         );

src/options.ts

@@ -64,7 +64,9 @@ function restoreOptions() {
       separator: '\u200B',
     },
     items => {
-      setSeparator(items.separator);
+      if (typeof items.separator === 'string') {
+        setSeparator(items.separator);
+      }
     }
   );
 }