Fixed NPE due to TNode processing before CNode

[andsel]

This PR added accessors methods to topic, in this way the TNode could throw exception if it's called. This let us fix #505 and #529, implementing the suggestion in #505 to move forward the TNode check.

Close #505
Close #529

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How to test
usually the error appeared with a couple of run of:

sudo docker run --rm --network host inovex/mqtt-stresser -broker tcp://localhost:1883 -num-clients 40 -num-messages 150 -rampup-delay 1s -rampup-size 10 -global-timeout 180s -timeout 20s