Environment

✔ Getting NativeScript components versions information...
✔ Component nativescript has 6.7.8 version and is up to date.
✔ Component @nativescript/core has 6.5.12 version and is up to date.
✔ Component tns-android has 6.5.3 version and is up to date.
✔ Component tns-ios has 6.5.2 version and is up to date.

Describe the bug
When running the app, Android only, we connectivity.startMonitoring() on app resume, and stop monitoring on the other hooks, typically "suspend". Android suspends when the app in minimized, when asking for permissions, when showing a camera or filepicker, etc.

Sometimes, but not all the time, the onAvailable() or onCapabilitiiesChanged() does not have a valid reference inside the connectivty module and it dies. Related to this area of code and here is a stack trace:

JS: Crash Message: Calling js method onCapabilitiesChanged failed
JS: TypeError: notifyCallback is not a function
JS: Stack: onCapabilitiesChanged(file: node_modules/@nativescript/core/connectivity/connectivity.android.js:108:24)
JS: 	at com.tns.Runtime.callJSMethodNative(Native Method)
JS: 	at com.tns.Runtime.access$2300(Runtime.java:39)
JS: 	at com.tns.Runtime$5.run(Runtime.java:1307)
JS: 	at android.os.Handler.handleCallback(Handler.java:883)
JS: 	at android.os.Handler.dispatchMessage(Handler.java:100)
JS: 	at android.os.Looper.loop(Looper.java:241)
JS: 	at android.app.ActivityThread.main(ActivityThread.java:7604)
JS: 	at java.lang.reflect.Method.invoke(Native Method)
JS: 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
JS: 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:941)

To Reproduce
Minimize and Open app repeatedly.
Or sometimes have phone sleep while app is open, and unlocking phone causes the bad timing (like the monitor was stopped but the callback for onCapabilitiesChanges still fired)

Expected behavior
Not crashing. If the callback for startMonitoring fires and there is no notifyCallback (because it was destroyed on stopMonitoring), then fail more gracefully.

Sample project
This project is not a working sample. (Namely I have not made permissions work yet for the camera). But it does replicate the suspend and resume when minimizing and opening the app. But it does not crash! Whatever else our real app is doing on "resume" must cause the bad timing.
https://play.nativescript.org/?template=play-vue&id=vaxKwu&v=3

Additional context
I'd gladly put a "mutex" in my code but that monitoring callback is outside of my control. If the callback is wanting to be fired but I stopped monitoring already, there is no callback and it crashes. I could try a PR if just defensive coding is helpful here

if (notifyCallback) notifyCallback()

but realize there could be more to this connectivity module.