ManagerV2 panic on stopping output unit and TestManagerV2 bad assertion

Apparently TestManagerV2 assertions are not done as expected. allStopped is called at the very beginning of the test instead of at last. When attempting to fix it,

diff --git a/x-pack/libbeat/management/managerV2_test.go b/x-pack/libbeat/management/managerV2_test.go
index f1b32b15d8..767be99b97 100644
--- a/x-pack/libbeat/management/managerV2_test.go
+++ b/x-pack/libbeat/management/managerV2_test.go
@@ -70,7 +70,7 @@ func TestManagerV2(t *testing.T) {
                        if oCfg == nil && len(iCfgs) == 0 && apmCfg == nil {
                                configsCleared.Store(true)
                        }
-                       if len(observed.Units) == 0 {
+                       if len(observed.Units) == 0 && currentIdx == 3 {
                                allStopped.Store(true)
                                t.Log("output and inputs configuration cleared (stopping)")
                        }

It generates a panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x4 pc=0xc37369]

goroutine 16 [running]:
github.com/elastic/beats/v7/x-pack/libbeat/management.(*agentUnit).UpdateState(0x0, 0x3, {0xe26514, 0x7}, 0x0)
    /home/carson/projects/beats/x-pack/libbeat/management/unit.go:252 +0x69
github.com/elastic/beats/v7/x-pack/libbeat/management.(*BeatV2Manager).reload(0xc0002f4240, 0xc000051e58)
    /home/carson/projects/beats/x-pack/libbeat/management/managerV2.go:672 +0x9f1
github.com/elastic/beats/v7/x-pack/libbeat/management.(*BeatV2Manager).unitListen(0xc0002f4240)
    /home/carson/projects/beats/x-pack/libbeat/management/managerV2.go:548 +0x7fc
created by github.com/elastic/beats/v7/x-pack/libbeat/management.(*BeatV2Manager).Start in goroutine 7
    /home/carson/projects/beats/x-pack/libbeat/management/managerV2.go:296 +0x1e5

This is due to a missing outputUnit nil check in https://github.com/elastic/beats/blob/367d94e1dc40a82713d651735eac7921bb584474/x-pack/libbeat/management/managerV2.go#L672 . outputUnit is nil because the unit is stopping. It is unclear whether this panic is possible in real scenarios, especially in cases where output can be hot reloaded, e.g. apm-server.

There are 2 parts to this issue:

Investigate and fix panic on stopping output unit
Fix assertion in TestManagerV2

elastic / beats

ManagerV2 panic on stopping output unit and TestManagerV2 bad assertion #41796