getSelectionButtonConfigForTenant(tenantId) {
    const uiConfigs = this.config_.get('tenants');
    if (!uiConfigs) {
      throw new Error('Invalid tenant configuration!');
    }
    const tenantConfig =
        uiConfigs[tenantId] ||
        uiConfigs[UiHandlerConfig.ConfigKeys.DEFAULT_CONFIG_KEY];
    if (!tenantConfig) {
      log.error(`Invalid tenant configuration: `+
                `${tenantId} is not configured!`);
      return null;
    }
    // The button config key cannot have the quote mark, since the key name will
    // be renamed in the soy template.
    return {
      tenantId: tenantId !== UiHandlerConfig.ConfigKeys.TOP_LEVEL_CONFIG_KEY ?
          tenantId : null,
      fullLabel: tenantConfig['fullLabel'] || null,
      displayName: tenantConfig['displayName'],
      iconUrl: tenantConfig['iconUrl'],
      buttonColor: tenantConfig['buttonColor'],
    };
  }
Returns the tenant selection button config object for the option first
tenant selection screen. Returns null if tenant button is not configured.
@param {string} tenantId The tenant ID or project-level config key.
@return {?Object} The tenant selection button config object for the tenant.