Fix: Prevent crash in Document.prototype.init() with null/undefined

lib/document.js

@@ -677,6 +677,9 @@ Document.prototype.$init = function() {
  */
 
 Document.prototype.$__init = function(doc, opts) {
+  if (doc == null) {
+    throw new ObjectParameterError(doc, 'doc', 'init');
+  }
   this.$isNew = false;
   opts = opts || {};
 

test/gh-15812.test.js

@@ -0,0 +1,32 @@
+'use strict';
+
+const start = require('./common');
+const mongoose = start.mongoose;
+const assert = require('assert');
+const ObjectParameterError = require('../lib/error/objectParameter');
+
+describe('gh-15812', function () {
+    it('should throw ObjectParameterError when init is called with null', function () {
+        const doc = new mongoose.Document({}, new mongoose.Schema({ name: String }));
+        try {
+            doc.init(null);
+            assert.fail('Should have thrown an error');
+        } catch (error) {
+            assert.ok(error instanceof ObjectParameterError);
+            assert.strictEqual(error.name, 'ObjectParameterError');
+            assert.ok(error.message.includes('Parameter "doc" to init() must be an object'));
+        }
+    });
+
+    it('should throw ObjectParameterError when init is called with undefined', function () {
+        const doc = new mongoose.Document({}, new mongoose.Schema({ name: String }));
+        try {
+            doc.init(undefined);
+            assert.fail('Should have thrown an error');
+        } catch (error) {
+            assert.ok(error instanceof ObjectParameterError);
+            assert.strictEqual(error.name, 'ObjectParameterError');
+            assert.ok(error.message.includes('Parameter "doc" to init() must be an object'));
+        }
+    });
+});