InfluxDB fails to collect Sonarqube data with the following error:
[InfluxDB Plugin] Failed to collect data. Ignoring Exception:net.sf.json.JSONException: A JSONObject text must begin with '{' at character 0 of

I've looked at the code and it seems to be the following issue:
SonarQubePointGenerator.java line 82:

url = build.getEnvironment(listener).get("SONAR_HOST_URL");

On my machine (and at work) this call always results in a null value.

On line 86 the following check is done:

if (url != "") {
    this.sonarServer = url;
} else {
    this.sonarServer = sonarBuildLink.substring(0,
    sonarBuildLink.indexOf("/dashboard/index/" + this.sonarProjectName));
}

However, null is not equal to "" so this.sonarServer = url is always called. Therefore the data collection doesn't pull through because the endpoint is set at something like:

null/api/measures/component?metricKeys=ncloc,complexity,violations&componentKey=

which isn't a valid url.

Changing if (url != "") to if (url != null && url != "") fixed the issue for me.