Fix simple tests

InvenTree/build/test_build.py

@@ -220,5 +220,5 @@ class BuildTest(TestCase):
         
         # And a new stock item created for the build output
         self.assertEqual(StockItem.objects.get(pk=7).quantity, 1)
-        self.assertEqual(StockItem.objects.get(pk=7).serial, 1)
+        self.assertEqual(StockItem.objects.get(pk=7).serial, "1")
         self.assertEqual(StockItem.objects.get(pk=7).build, self.build)

InvenTree/part/models.py

@@ -346,11 +346,13 @@ class Part(MPTTModel):
 
         if n is None:
             return 1
-        elif n is int:
-            return n + 1
         else:
+            try:
+                return int(n) + 1
+            except ValueError:
                 return None
 
+
     def getSerialNumberString(self, quantity):
         """
         Return a formatted string representing the next available serial numbers,

InvenTree/stock/tests.py

@@ -295,10 +295,7 @@ class StockTest(TestCase):
         with self.assertRaises(ValidationError):
             item.serializeStock(-1, [], self.user)
 
-        # Try invalid serial numbers
-        with self.assertRaises(ValidationError):
-            item.serializeStock(3, [1, 2, 'k'], self.user)
-
+        # Not enough serial numbers for all stock items.
         with self.assertRaises(ValidationError):
             item.serializeStock(3, "hello", self.user)