message: Monitoring: Fix JS warning on Silence form setting state before mount
  Fix "Can't call setState on a component that is not yet mounted"
  JS warning.
diff: @@ -841,11 +841,10 @@ class SilenceForm_ extends React.Component<SilenceFormProps, SilenceFormState> {
  const startsAt = formatDate(now);
  const endsAt = formatDate(new Date(now.setHours(now.getHours() + 2)));
  const data = _.defaults(props.defaults, {startsAt, endsAt, matchers: [], createdBy: '', comment: ''});
  - this.state = {data, error: undefined, inProgress: false};
  -
  if (_.isEmpty(data.matchers)) {
  - this.addMatcher();
  + data.matchers.push({name: '', value: '', isRegex: false});
  }
  + this.state = {data, error: undefined, inProgress: false};
  }
  setField = (path: string, v: any): void => {
  