Fix nil pointer panic in ICE agent during sleep/wake cycles (#5261)

Add defensive nil checks in ThreadSafeAgent.Close() to prevent panic
when agent field is nil. This can occur during Windows suspend/resume
when network interfaces are disrupted or the pion/ice library returns
nil without error.

Also capture agent pointer in local variable before goroutine execution
to prevent race conditions.

Fixes service crashes on laptop wake-up.

client/internal/peer/ice/agent.go

@@ -2,6 +2,7 @@ package ice
 
 import (
 	"context"
+	"fmt"
 	"sync"
 	"time"
 
@@ -32,24 +33,6 @@ type ThreadSafeAgent struct {
 	once sync.Once
 }
 
-func (a *ThreadSafeAgent) Close() error {
-	var err error
-	a.once.Do(func() {
-		done := make(chan error, 1)
-		go func() {
-			done <- a.Agent.Close()
-		}()
-
-		select {
-		case err = <-done:
-		case <-time.After(iceAgentCloseTimeout):
-			log.Warnf("ICE agent close timed out after %v, proceeding with cleanup", iceAgentCloseTimeout)
-			err = nil
-		}
-	})
-	return err
-}
-
 func NewAgent(ctx context.Context, iFaceDiscover stdnet.ExternalIFaceDiscover, config Config, candidateTypes []ice.CandidateType, ufrag string, pwd string) (*ThreadSafeAgent, error) {
 	iceKeepAlive := iceKeepAlive()
 	iceDisconnectedTimeout := iceDisconnectedTimeout()
@@ -93,9 +76,41 @@ func NewAgent(ctx context.Context, iFaceDiscover stdnet.ExternalIFaceDiscover, c
 		return nil, err
 	}
 
+	if agent == nil {
+		return nil, fmt.Errorf("ice.NewAgent returned nil agent without error")
+	}
+
 	return &ThreadSafeAgent{Agent: agent}, nil
 }
 
+func (a *ThreadSafeAgent) Close() error {
+	var err error
+	a.once.Do(func() {
+		// Defensive check to prevent nil pointer dereference
+		// This can happen during sleep/wake transitions or memory corruption scenarios
+		// github.com/netbirdio/netbird/client/internal/peer/ice.(*ThreadSafeAgent).Close(0x40006883f0?)
+		//  [signal 0xc0000005 code=0x0 addr=0x0 pc=0x7ff7e73af83c]
+		agent := a.Agent
+		if agent == nil {
+			log.Warnf("ICE agent is nil during close, skipping")
+			return
+		}
+
+		done := make(chan error, 1)
+		go func() {
+			done <- agent.Close()
+		}()
+
+		select {
+		case err = <-done:
+		case <-time.After(iceAgentCloseTimeout):
+			log.Warnf("ICE agent close timed out after %v, proceeding with cleanup", iceAgentCloseTimeout)
+			err = nil
+		}
+	})
+	return err
+}
+
 func GenerateICECredentials() (string, string, error) {
 	ufrag, err := randutil.GenerateCryptoRandomString(lenUFrag, runesAlpha)
 	if err != nil {

client/internal/peer/worker_ice.go

@@ -107,8 +107,10 @@ func (w *WorkerICE) OnNewOffer(remoteOfferAnswer *OfferAnswer) {
 		}
 		w.log.Debugf("agent already exists, recreate the connection")
 		w.agentDialerCancel()
-		if err := w.agent.Close(); err != nil {
-			w.log.Warnf("failed to close ICE agent: %s", err)
+		if w.agent != nil {
+			if err := w.agent.Close(); err != nil {
+				w.log.Warnf("failed to close ICE agent: %s", err)
+			}
 		}
 
 		sessionID, err := NewICESessionID()