Fix the issue when plain part not found

javascript/src/helpers/mailparser.js

@@ -190,7 +190,7 @@ function getBody(mime, part) {
 }
 
 export function decodeMimeBody(descriptor, mimeString) {
-  let mimeBody = mimeString.slice(
+  const mimeBody = mimeString.slice(
     descriptor.indices.headersEnd,
     descriptor.indices.to
   );
@@ -300,7 +300,9 @@ export function getPlain(mime, parts) {
       break;
     }
   }
-
+  if (!plainPart) {
+    return "";
+  }
   plain = decodeMimeBody(plainPart, mime);
   return plain;
 }