Cannot add issue/pull dependencies

[Larivact]

Reproducible at https://try.gitea.io (currently dbb4aee).

Description

1. Create a repository.
2. Create two issues for your repository.
3. Open one issue and pick the other in the Dependencies select. Click the green plus.
4. The site reloads and there are still no dependencies.

[InExtremaRes]

The same is happening with PRs, not only issues.

[SlavekB]

In the log I see a lot of reports related to this problem:

[Macaron] 2019-01-24 14:36:54: Started POST /TDE/tde-common-cmake/issues/20/dependency/add for _client_address_
[Macaron] PANIC: runtime error: invalid memory address or nil pointer dereference
 /usr/local/go/src/runtime/panic.go:513 (0x430db8)
 /usr/local/go/src/runtime/panic.go:82 (0x42ff0d)
 /usr/local/go/src/runtime/signal_unix.go:390 (0x446e31)
 /go/src/code.gitea.io/gitea/models/repo.go:409 (0xef77c6)
 /go/src/code.gitea.io/gitea/models/issue_comment.go:529 (0xeaaf57)
 /go/src/code.gitea.io/gitea/models/issue_comment.go:753 (0xeacb28)
 /go/src/code.gitea.io/gitea/models/issue_dependency.go:66 (0xeb10bf)
 /go/src/code.gitea.io/gitea/routers/repo/issue_dependency.go:54 (0x106911a)
 /usr/local/go/src/runtime/asm_amd64.s:522 (0x45f73a)
 /usr/local/go/src/reflect/value.go:447 (0x4c71c8)
 /usr/local/go/src/reflect/value.go:308 (0x4c6c63)
 /go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/inject/inject.go:177 (0x95f793)
 /go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/inject/inject.go:137 (0x95f0f9)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/context.go:121 (0x98d280)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/context.go:112 (0x98d19e)
 /go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/session/session.go:188 (0x9e0f3e)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/context.go:79 (0x98d120)
 /go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/inject/inject.go:157 (0x95f480)
 /go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/inject/inject.go:135 (0x95f1eb)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/context.go:121 (0x98d280)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/context.go:112 (0x98d19e)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/recovery.go:161 (0x99df5a)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/logger.go:40 (0x990423)
 /go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/inject/inject.go:157 (0x95f480)
 /go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/inject/inject.go:135 (0x95f1eb)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/context.go:121 (0x98d280)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/context.go:112 (0x98d19e)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/logger.go:52 (0x99d270)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/logger.go:40 (0x990423)
 /go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/inject/inject.go:157 (0x95f480)
 /go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/inject/inject.go:135 (0x95f1eb)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/context.go:121 (0x98d280)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/router.go:187 (0x99f154)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/router.go:303 (0x998aae)
 /go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/macaron.go:220 (0x991711)
 /go/src/code.gitea.io/gitea/vendor/github.com/gorilla/context/context.go:141 (0xcfca9a)
 /usr/local/go/src/net/http/server.go:1964 (0x6bf123)
 /usr/local/go/src/net/http/server.go:2741 (0x6c1e1a)
 /usr/local/go/src/net/http/server.go:1847 (0x6be185)
 /usr/local/go/src/runtime/asm_amd64.s:1333 (0x461450)
 2019/01/24 14:36:54 http: multiple response.WriteHeader calls
[Macaron] 2019-01-24 14:36:54: Completed POST /TDE/tde-common-cmake/issues/20/dependency/add 500 Internal Server Error in 179.590241ms