[BUG] - "call of reflect.Value.Type on zero Value" error when using in SingletonMode

[Jonas096]

Describe the bug

When using the SingletonMode the module panics with the error "call of reflect.Value.Type on zero Value". The weird thing is, this error applys only 3 of 4 times. Sometimes the applications starts well.

To Reproduce

My Setup is like this

`func InitScheduler() {
gocron.SetPanicHandler(func(jobName string, recoverData interface{}) {
logs.Logger.Panicln("jobName", jobName, "error", recoverData)
})
Scheduler = gocron.NewScheduler(time.UTC)

//maximal werden 5 Jobs gleichzeitig durchgeführt
//jeder Job danach wird einfach in eine Warteschlange gesetzt
Scheduler.SetMaxConcurrentJobs(5, gocron.WaitMode)

//wenn der Job noch läuft, und eigentlich jetzt wieder an der Zeit durchgeführt werden sollte
//wird er nicht wieder ausgeführt
Scheduler.SingletonMode()

defer Scheduler.StartAsync()

}`

Version

go 1.18
github.com/go-co-op/gocron v1.16.2

Expected behavior

No Panic at start of the application

[jerebtw]

I also have this bug

[JohnRoesler]

@jerebtw @Jonas096 sorry for the late reply!

Scheduler.SingletonMode() is meant to be called in a chain defining a job. For example:

Scheduler.Every(10).Second().SingletonMode().Do(func() {
	log.Println("hello")
})

The global scheduler option is:

Scheduler = gocron.NewScheduler(time.UTC)
Scheduler.SetMaxConcurrentJobs(5, gocron.WaitMode)
Scheduler.SingletonModeAll()

This isn't handled well currently as you've encountered. I'll push up a fix to catch this situation.