fix(model): avoid uncaught error if insertMany() fails due to serve…

…r selection error

Fix #9355

lib/model.js

@@ -3323,7 +3323,7 @@ Model.$__insertMany = function(arr, options, callback) {
         }
 
         // `insertedDocs` is a Mongoose-specific property
-        const erroredIndexes = new Set(error.writeErrors.map(err => err.index));
+        const erroredIndexes = new Set(get(error, 'writeErrors', []).map(err => err.index));
         error.insertedDocs = docAttributes.filter((doc, i) => {
           return !erroredIndexes.has(i);
         });