Here's the offending lines:

Boolean enabled = Boolean.valueOf(logEventSource.getSimpleValue(LogEventSourcePropertyNames.ENABLED, null));
            if (enabled) {

The NPE could occur at the if-check if a plugin set the 'enabled' prop to null or did not initialize it at all. 

The code should be changed to:

Boolean enabled = Boolean.valueOf(logEventSource.getSimpleValue(LogEventSourcePropertyNames.ENABLED, null));
            if (enabled == null) {
               String logFilePath =  logEventSource.getSimpleValue(LogEventSourcePropertyNames.LOG_FILE_PATH, null);
               log.warn("Plugin error: 'enabled' property is null for event source for log file [" + logFilePath + "] - this property is required and should always be set. Assuming the event source is disabled...");
                enabled = Boolean.FALSE;
            }
            if (enabled) {