Steps to reproduce

------------------

1. Install Zabbix Agent 2 8.0.0rc1-2+ubuntu24.04 on Ubuntu 24.04 LTS amd64.

2. Use the standard Agent 2 configuration with:

Include=/etc/zabbix/zabbix_agent2.d/plugins.d/*.conf

3. Do not define MQTT Default.ConnectionTimeout.

4. Start Zabbix Agent 2.

Actual result

-------------

The Agent terminates during startup with:

panic: runtime error: invalid memory address or nil pointer dereference

The stack trace points to:

golang.zabbix.com/agent2/plugins/mqtt.(*Plugin).Configure

src/go/plugins/mqtt/config.go:62

The same crash can be reproduced manually:

/usr/sbin/zabbix_agent2 -f -c /etc/zabbix/zabbix_agent2.conf

The crash occurs during MQTT plugin initialization.

No active MQTT configuration is required to reproduce the problem.

Impact

------

The MQTT plugin crash terminates the entire Zabbix Agent 2 process.

As a result, all Agent-based monitoring becomes unavailable, including unrelated monitoring such as Modbus TCP.

The Modbus configuration itself was not changed.

Workaround

----------

Adding the following parameter to mqtt.conf immediately resolves the problem:

Plugins.MQTT.Default.ConnectionTimeout=5

After adding this single parameter, Zabbix Agent 2 starts normally and monitoring is immediately restored, including Modbus TCP.

No MQTT broker connection is required for the workaround.

Expected result

---------------

Zabbix Agent 2 should start normally when no MQTT Default configuration is specified.

The absence of optional MQTT configuration should not cause the entire Agent 2 process to terminate with a nil pointer dereference.

The MQTT plugin should safely initialize its Default configuration and use the global/default timeout when no explicit ConnectionTimeout is configured.

Additional information

----------------------

The problem appeared after updating to Zabbix Agent 2 8.0.0rc1.

Adding Plugins.MQTT.Default.ConnectionTimeout=5 was tested on the affected system and immediately restored normal Agent 2 operation.