Getting a reflection panic when trying to save a record with a belongs_to pointer

[akostibas]

Description

We are trying to update from 4.8.7 to 4.9.0.

I have a model that uses pointers for relational fields that are optional (nullable.) For example:

type TransportationOffice struct {
    ID               uuid.UUID             `json:"id" db:"id"`                                             
    ShippingOfficeID *uuid.UUID            `json:"shipping_office_id" db:"shipping_office_id"`             
    ShippingOffice   *TransportationOffice `belongs_to:"transportation_offices"`
    Name             string                `json:"name" db:"name"`
    Address          Address               `belongs_to:"address"`
    ...

Full source here

Now, when trying to save this model with a null value in the ShippingOffice field, I'm getting a panic in the reflection package:

panic: reflect: call of reflect.Value.FieldByName on zero Value [recovered]
	panic: reflect: call of reflect.Value.FieldByName on zero Value

goroutine 66 [running]:
testing.tRunner.func1(0xc42058c000)
	/usr/local/Cellar/go@1.10/1.10.4/libexec/src/testing/testing.go:742 +0x29d
panic(0x18388a0, 0xc420671320)
	/usr/local/Cellar/go@1.10/1.10.4/libexec/src/runtime/panic.go:502 +0x229
reflect.flag.mustBe(0x0, 0x19)
	/usr/local/Cellar/go@1.10/1.10.4/libexec/src/reflect/value.go:207 +0xb5
reflect.Value.FieldByName(0x0, 0x0, 0x0, 0x191afb0, 0x2, 0x0, 0x0, 0x0)
	/usr/local/Cellar/go@1.10/1.10.4/libexec/src/reflect/value.go:835 +0x4c
github.com/transcom/mymove/vendor/github.com/gobuffalo/pop/associations.(*belongsToAssociation).BeforeSetup(0xc42066e300, 0x185b940, 0x0)
	/Users/alexi/Documents/projects/golang/src/github.com/transcom/mymove/vendor/github.com/gobuffalo/pop/associations/belongs_to_association.go:113 +0x13e
github.com/transcom/mymove/vendor/github.com/gobuffalo/pop.(*Connection).Create.func1.1(0xbef2a84560ba7cae, 0x2a399c4b)
	/Users/alexi/Documents/projects/golang/src/github.com/transcom/mymove/vendor/github.com/gobuffalo/pop/executors.go:136 +0x8b7
github.com/transcom/mymove/vendor/github.com/gobuffalo/pop.(*Connection).timeFunc(0xc4200f53e0, 0x191eacb, 0x6, 0xc420d2b790, 0xc42046b680, 0xc420d2b7c8)
	/Users/alexi/Documents/projects/golang/src/github.com/transcom/mymove/vendor/github.com/gobuffalo/pop/connection.go:206 +0x50
github.com/transcom/mymove/vendor/github.com/gobuffalo/pop.(*Connection).Create.func1(0xc42052ba70, 0x23ab400, 0x0)
	/Users/alexi/Documents/projects/golang/src/github.com/transcom/mymove/vendor/github.com/gobuffalo/pop/executors.go:112 +0xa7
github.com/transcom/mymove/vendor/github.com/gobuffalo/pop.(*Model).iterate(0xc42052ba70, 0xc42052bad0, 0xc420d2b898, 0x156db9a)
	/Users/alexi/Documents/projects/golang/src/github.com/transcom/mymove/vendor/github.com/gobuffalo/pop/model.go:197 +0x20b
github.com/transcom/mymove/vendor/github.com/gobuffalo/pop.(*Connection).Create(0xc4200f53e0, 0x18fc7c0, 0xc42046b680, 0x0, 0x0, 0x0, 0x0, 0xc420d2b980)
	/Users/alexi/Documents/projects/golang/src/github.com/transcom/mymove/vendor/github.com/gobuffalo/pop/executors.go:111 +0xf0
github.com/transcom/mymove/vendor/github.com/gobuffalo/pop.(*Connection).ValidateAndCreate(0xc4200f53e0, 0x18fc7c0, 0xc42046b680, 0x0, 0x0, 0x0, 0x0, 0xc4200a4000, 0xc420d2b980)
	/Users/alexi/Documents/projects/golang/src/github.com/transcom/mymove/vendor/github.com/gobuffalo/pop/executors.go:100 +0x111
github.com/transcom/mymove/pkg/testdatagen.mustCreate(0xc4200f53e0, 0x18fc7c0, 0xc42046b680)
	/Users/alexi/Documents/projects/golang/src/github.com/transcom/mymove/pkg/testdatagen/shared.go:70 +0x6b
github.com/transcom/mymove/pkg/testdatagen.MakeTransportationOffice(0xc4200f53e0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/Users/alexi/Documents/projects/golang/src/github.com/transcom/mymove/pkg/testdatagen/make_transportation_office.go:24 +0x21d

It looks like some work was done in/around the belongs_to_association.go code recently: #199 that may be related.

Happy to provide more details if necessary.