using external type results in panic

[jiyoonie9]

current behavior

in module A, i have this struct

type DetailResponse {
   id       string
   token token.Detail // is an external type, TypeID
   ...
}

and i built an ftl verb that returns this verb

//ftl:verb export
func GetDetails(ctx context.Context, req DetailRequest) (DetailResponse, error) {
   ...

}

when i try to do ftl build backend/modules/A i get the following stacktrace

project:shell: *[my-branch][~/IdeaProjects/MyProject]$ ftl build backend/modules/A
info:A: Building module
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x30 pc=0x1046c6300]

goroutine 82 [running]:
github.com/TBD54566975/ftl/backend/schema.Visit.func1()
        github.com/TBD54566975/ftl/backend/schema/visit.go:8 +0x20
github.com/TBD54566975/ftl/go-runtime/compile.updateTransitiveVisibility.func1({0x0?, 0x0?}, 0x14000b24a80?)
        github.com/TBD54566975/ftl/go-runtime/compile/build.go:661 +0xe8
github.com/TBD54566975/ftl/backend/schema.Visit({0x0, 0x0}, 0x140002e52a0)
        github.com/TBD54566975/ftl/backend/schema/visit.go:7 +0x8c
github.com/TBD54566975/ftl/backend/schema.Visit.func1()
        github.com/TBD54566975/ftl/backend/schema/visit.go:9 +0x68
github.com/TBD54566975/ftl/go-runtime/compile.updateTransitiveVisibility.func1({0x105f59b38?, 0x14000d043f0?}, 0x140006cfe01?)
        github.com/TBD54566975/ftl/go-runtime/compile/build.go:661 +0xe8
github.com/TBD54566975/ftl/backend/schema.Visit({0x105f59b38, 0x14000d043f0}, 0x140002e52a0)
        github.com/TBD54566975/ftl/backend/schema/visit.go:7 +0x8c
github.com/TBD54566975/ftl/go-runtime/compile.updateTransitiveVisibility({0x105f669d8, 0x14000d043f0}, 0x140002ff180)
        github.com/TBD54566975/ftl/go-runtime/compile/build.go:658 +0xb4
github.com/TBD54566975/ftl/go-runtime/compile.updateVisibility(0x140002ff180)
        github.com/TBD54566975/ftl/go-runtime/compile/build.go:647 +0x78
github.com/TBD54566975/ftl/go-runtime/compile.ExtractModuleSchema({0x14000436c40, 0x38}, 0x140006022c0)
        github.com/TBD54566975/ftl/go-runtime/compile/build.go:639 +0x164
github.com/TBD54566975/ftl/go-runtime/compile.Build({0x105f59278, 0x1400003bd10}, {0x14000436c40, 0x38}, 0x140006022c0, {0x12dcf98a0, 0x1400047b700})
        github.com/TBD54566975/ftl/go-runtime/compile/build.go:144 +0x4e4
github.com/TBD54566975/ftl/buildengine.buildGoModule({0x105f59278?, 0x1400003bd10?}, 0x140006022c0?, {{{0x14000436c40, 0x38}, {0x140004c3278, 0x2}, {0x140004c327c, 0x4}, {0x140004c3280, ...}, ...}, ...}, ...)
        github.com/TBD54566975/ftl/buildengine/build_go.go:13 +0x70
github.com/TBD54566975/ftl/buildengine.buildModule({0x105f59278, 0x1400003bcb0}, 0x140006022c0, {{{0x14000436c40, 0x38}, {0x140004c3278, 0x2}, {0x140004c327c, 0x4}, {0x140004c3280, ...}, ...}, ...}, ...)
        github.com/TBD54566975/ftl/buildengine/build.go:58 +0x560
github.com/TBD54566975/ftl/buildengine.Build({0x105f59278?, 0x1400003bcb0?}, 0x14000127a40?, {0x105f59ba8?, 0x1400044e270?}, {0x105f54390?, 0x1400047b700?})
        github.com/TBD54566975/ftl/buildengine/build.go:33 +0xa4
github.com/TBD54566975/ftl/buildengine.(*Engine).build(0x14000146990, {0x105f59278, 0x1400003bcb0}, {0x140003fcdc0, 0x9}, 0x140004b6630, 0x14000450360)
        github.com/TBD54566975/ftl/buildengine/engine.go:569 +0x3dc
github.com/TBD54566975/ftl/buildengine.(*Engine).tryBuild(0x14000146990, {0x105f59278, 0x1400003bcb0}, 0x140005b2210, {0x140003fcdc0, 0x9}, 0x140004b6630, 0x14000450360, 0x0)
        github.com/TBD54566975/ftl/buildengine/engine.go:534 +0x2f4
github.com/TBD54566975/ftl/buildengine.(*Engine).buildWithCallback.func2()
        github.com/TBD54566975/ftl/buildengine/engine.go:480 +0x3a4
golang.org/x/sync/errgroup.(*Group).Go.func1()
        golang.org/x/[email protected]/errgroup/errgroup.go:78 +0x58
created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 1
        golang.org/x/[email protected]/errgroup/errgroup.go:75 +0x98

expected behavior

i would expect to ftl to give me a helpful error that says external type is not supported, something like this.

error: build and deploy failed for module "echo": /.../ftl/examples/go/echo/echo.go:29:30-36: unsupported type "invalid type" for field "TypeID"
/.../ftl/examples/go/echo/echo.go:37:2-10: unsupported type "ftl/echo.Customer" for field "Customer"
/.../ftl/examples/go/echo/echo.go:43:50-50: unsupported response type "ftl/echo.EchoResponse"

other notes

we were not able to recreate this error when using the echo module inside ftl repo's example directory.