fix(validations): fix named export

As community may sometimes be null, I edited the
isDisputeSolver function to accept null, so we don't have to
do a null check over it.

```
bot/validations.ts:97:38 - error TS2345: Argument of type '(ICommunity & { _id: ObjectId; }) | null' is not assignable to parameter of type 'ICommunity'.
  Type 'null' is not assignable to type 'ICommunity'.

97     const isSolver = isDisputeSolver(community, user);
                                        ~~~~~~~~~

Found 1 error in bot/validations.ts:97
```

bot/validations.ts

@@ -6,7 +6,7 @@ import { IOrder } from "../models/order";
 import { Telegraf } from "telegraf";
 
 const { parsePaymentRequest } = require('invoices');
-const { ObjectId } = require('mongoose').Types;
+import { Types } from 'mongoose';
 import * as messages from './messages';
 import { Order, User, Community } from '../models';
 import { isIso4217, isDisputeSolver } from '../util';
@@ -94,7 +94,6 @@ const validateAdmin = async (ctx: MainContext, id?: string) => {
     if (user.default_community_id)
       community = await Community.findOne({ _id: user.default_community_id });
 
-    if (community === null) throw Error("Community was not found in DB");
     const isSolver = isDisputeSolver(community, user);
 
     if (!user.admin && !isSolver)
@@ -593,7 +592,7 @@ const validateParams = async (ctx: MainContext, paramNumber: number, errOutputSt
 
 const validateObjectId = async (ctx: MainContext, id: string) => {
   try {
-    if (!ObjectId.isValid(id)) {
+    if (!Types.ObjectId.isValid(id)) {
       await messages.notValidIdMessage(ctx);
       return false;
     }
@@ -647,7 +646,7 @@ const isBannedFromCommunity = async (user: UserDocument, communityId: string) =>
   }
 };
 
-module.exports = {
+export {
   validateSellOrder,
   validateBuyOrder,
   validateUser,

util/index.ts

@@ -382,7 +382,7 @@ const getDetailedOrder = (i18n: I18nContext, order: IOrder, buyer: UserDocument,
 };
 
 // We need to know if this user is a dispute solver for this community
-const isDisputeSolver = (community: ICommunity, user: UserDocument) => {
+const isDisputeSolver = (community: ICommunity | null, user: UserDocument) => {
   if (!community || !user) {
     return false;
   }