Cannot read property 'ownerColumns' of undefined

Issue type:

[ ] question [x] bug report [ ] feature request [ ] documentation issue

Database system/driver:

[ ] cordova [ ] mongodb [ ] mssql [x] mysql / mariadb [ ] oracle [ ] postgres [ ] sqlite [ ] sqljs [ ] react-native [ ] expo

TypeORM version:

[x] latest [ ] @next [ ] 0.x.x (or put your version here)

Steps to reproduce or a small repository showing the problem:

Problems listed below. Trying to save this causes the issue mentioned in the title

// Report.ts
import {BaseEntity, Column, Entity, Index, ManyToMany, PrimaryGeneratedColumn} from 'typeorm';
import ReportCategoryEnum from '../ReportCategory';
import User from './User';



@Entity('Report')
export default class Report extends BaseEntity {
    @PrimaryGeneratedColumn()
    public Id: number;

    @ManyToMany(type => User)
    public Reporter: User;

    @Column() @Index('category', ['Category'])
    public Category: ReportCategoryEnum;

    @Column({type: 'text'})
    public Reason: string;

    @Column({type: 'bigint'}) @Index('guild', ['GuildId'])
    public GuildId?: string;

    @Column({type: 'simple-array'})
    public MessageIds: string[] = [];

    @Column({type: 'simple-array'})
    public Links: string[];

    @ManyToMany(type => User)
    public ReportedUsers: User[];

    @ManyToMany(type => User)
    public ConfirmationUsers: User[];

    @Column({type: 'datetime'}) @Index('insert_date', ['InsertDate'])
    public InsertDate: Date = new Date;

    @Column({type: 'datetime'}) @Index('update_date', ['UpdateDate'])
    public UpdateDate: Date = new Date;
}

// User.ts
import {BaseEntity, Column, Entity, Index, ManyToMany, PrimaryColumn, PrimaryGeneratedColumn, Table} from 'typeorm';
import Report from './Report';

@Entity('User')
export default class User extends BaseEntity {
    @Column({type: 'bigint'}) @Index('user_id', ['UserId'])
    @PrimaryColumn()
    public Id: string;

    @Column({type: 'datetime'}) @Index('insert_date', ['InsertDate'])
    public InsertDate: Date = new Date;

    constructor(id: string) {
        super();

        this.Id = id;
    }
}

// Code calling this

const report    = new Report();
report.Reporter = await repo.findOne(body.Reporter) || new User(body.Reporter);
report.Category = body.Category;
report.Reason   = body.Reason;
report.GuildId  = body.GuildId;

const promises: Promise<any>[] = [];
for (const x of body.ReportedUsers) {
    const user = await repo.findOne(x) || new User(x);
    if (!report.ReportedUsers) {
        report.ReportedUsers = [];
    }

    report.ReportedUsers.push(user);
    promises.push(user.save())
}

await Promise.all([...promises, report.save()]);