Hi Arnaud,
I want to report a bug, would you help to check?

[Issue]: All Text Mapping and Value Mapping rules will be not working when a Value Mapping rule get the "null " metric value.
[Version]: 0.4.0
[Reproduce Steps]:
1. Create a panel and add some mapping rules.
2. Panel display normally.

3. Due to some database networking issue, grafana may query some metrics with "null" value sometimes. If this metric is bonding to a value mapping rule, then all rules will be not working. And there will an error cannot read property "toString" of null

[Desired result]: It should only affect the specific rule that have the problem, instead of broking all rules.