fix: a possible crash in event target Close() (#17948)

these are possible crashes when the configured
target is still in init() state and never finished
- however a delete config was initiated.

internal/event/target/mqtt.go

@@ -219,7 +219,9 @@ func (target *MQTTTarget) Save(eventData event.Event) error {
 
 // Close - does nothing and available for interface compatibility.
 func (target *MQTTTarget) Close() error {
-	target.client.Disconnect(100)
+	if target.client != nil {
+		target.client.Disconnect(100)
+	}
 	close(target.quitCh)
 	return nil
 }

internal/event/target/mysql.go

@@ -312,7 +312,11 @@ func (target *MySQLTarget) Close() error {
 		_ = target.insertStmt.Close()
 	}
 
-	return target.db.Close()
+	if target.db != nil {
+		return target.db.Close()
+	}
+
+	return nil
 }
 
 // Executes the table creation statements.

internal/event/target/postgresql.go

@@ -308,7 +308,11 @@ func (target *PostgreSQLTarget) Close() error {
 		_ = target.insertStmt.Close()
 	}
 
-	return target.db.Close()
+	if target.db != nil {
+		target.db.Close()
+	}
+
+	return nil
 }
 
 // Executes the table creation statements.

internal/event/target/redis.go

@@ -273,7 +273,10 @@ func (target *RedisTarget) SendFromStore(eventKey string) error {
 // Close - releases the resources used by the pool.
 func (target *RedisTarget) Close() error {
 	close(target.quitCh)
-	return target.pool.Close()
+	if target.pool != nil {
+		return target.pool.Close()
+	}
+	return nil
 }
 
 func (target *RedisTarget) init() error {