fix(connectivity_plus): guard eventSink against FlutterEngine teardown on iOS

NWPathMonitor can fire a connectivity update while the app is in the
background, after the FlutterEngine's shell has been torn down. The
DispatchQueue.main.async block then invokes the stale FlutterEventSink,
which calls -[FlutterEngine sendOnChannel:] and hits an NSAssertion at
FlutterEngine.mm:1315, aborting the process with SIGABRT.

Guard the main-queue emission with [weak self], a local eventSink copy,
and a UIApplication.applicationState != .background check so that
pending updates are dropped instead of propagated into a dead engine.
The next onListen / check call after foregrounding will resync state.

Author: Till Friebe

packages/connectivity_plus/connectivity_plus/ios/connectivity_plus/Sources/connectivity_plus/ConnectivityPlusPlugin.swift

@@ -3,6 +3,7 @@
 // be found in the LICENSE file.
 
 import Flutter
+import UIKit
 
 public class ConnectivityPlusPlugin: NSObject, FlutterPlugin, FlutterStreamHandler {
   private let connectivityProvider: ConnectivityProvider
@@ -81,8 +82,15 @@ public class ConnectivityPlusPlugin: NSObject, FlutterPlugin, FlutterStreamHandl
   }
 
   private func connectivityUpdateHandler(connectivityTypes: [ConnectivityType]) {
-    DispatchQueue.main.async {
-      self.eventSink?(self.statusFrom(connectivityTypes: connectivityTypes))
+    DispatchQueue.main.async { [weak self] in
+      guard let self = self, let eventSink = self.eventSink else { return }
+      // NWPathMonitor can fire while the app is in the background, after the
+      // FlutterEngine's shell has been torn down. Calling eventSink in that
+      // state triggers an NSAssertion in -[FlutterEngine sendOnChannel:] and
+      // crashes the app with SIGABRT. Skip the emission until the app is
+      // foregrounded again; the next onListen / check call will resync state.
+      guard UIApplication.shared.applicationState != .background else { return }
+      eventSink(self.statusFrom(connectivityTypes: connectivityTypes))
     }
   }