@appsignal/vue: errorHandler crashes on null instance and cannot resolve component names on Vue 3

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Package: @appsignal/vue 1.1.7
Vue version: 3.5.x

The errorHandler in @appsignal/vue has two problems when used with Vue 3:

1. Crash when Vue invokes the handler with a null instance

Vue 3 calls app.config.errorHandler(err, instance, info) with instance: null for errors outside a component context (e.g. errors in watchers or lifecycle hooks after unmount). The handler dereferences the instance unguarded:

const componentName = vm.$vnode
  ? vm.$vnode.componentOptions.tag // Vue 2
  : vm.$options.name || vm.$options.__name // Vue 3

With vm = null this throws TypeError: Cannot read properties of null (reading '$vnode') inside the error handler itself, so the original error is never reported to AppSignal and never reaches console.error.

2. Component name resolution effectively never works on Vue 3

• The vm.$vnode branch is Vue 2 API; on Vue 3 it is always undefined, so it is dead code.
• $options.name is only set for components that declare an explicit name option, which most codebases don't do (the SFC filename is usually the source of truth).
• $options.__name is only set for <script setup> components.

For classic Options API SFCs without an explicit name, none of the branches match, so every error is reported with the action [unknown Vue component]. In practice this means all frontend errors group under a single incident, which defeats per-component grouping.

Suggested fix

function componentName(vm) {
  const options = vm?.$options
  if (!options) return null
  const file = options.__file?.split("/").pop()?.replace(/\.vue$/, "")
  return options.name || options.__name || file || null
}

• Optional chaining fixes the null-instance crash.
• $options.__file is set by vue-loader (always in development; in production when the app opts in via vue-loader's exposeFilename option, which reduces it to the basename). It is a safe extra fallback: absent unless the app opted in, and the [unknown Vue component] fallback is preserved otherwise.

I've opened a PR with this change plus tests alongside this issue.