Fix null relation problem (parse-community#2319)

* Add null check for relation type map.

For relations that are not explicitly defined in the schema, we need a null check here.

* Making change to force rebuild.

* Reverting change.

* Adds test

spec/ParseUser.spec.js

@@ -2488,5 +2488,22 @@ describe('Parse.User testing', () => {
         })
       });
     });
-  })
+  });
+
+  it_exclude_dbs(['postgres'])('should not fail querying non existing relations', done => { 
+    let user = new Parse.User();
+    user.set({
+      username: 'hello',
+      password: 'world'
+    })
+    user.signUp().then(() => {
+      return Parse.User.current().relation('relation').query().find();
+    }).then((res) => {
+      expect(res.length).toBe(0);
+      done();
+    }).catch((err) => {
+      fail(JSON.stringify(err));
+      done();
+    });
+  });
 });

src/Controllers/DatabaseController.js

@@ -1,4 +1,4 @@
-// A database adapter that works with data exported from the hosted
+// A database adapter that works with data exported from the hosted
 // Parse database.
 
 import intersect from 'intersect';
@@ -121,7 +121,7 @@ DatabaseController.prototype.loadSchema = function() {
 DatabaseController.prototype.redirectClassNameForKey = function(className, key) {
   return this.loadSchema().then((schema) => {
     var t = schema.getExpectedType(className, key);
-    if (t.type == 'Relation') {
+    if (t && t.type == 'Relation') {
       return t.targetClass;
     } else {
       return className;