UnhandledPromiseRejectionWarning happening on Javascript database test

this is my second post here!
OK so here’s what happening, I’m learning MongoDB & more stuff about Javascript and me & my helper are completly stumped here.
I’m making an API for "Characters" and I’m checking if all the ID given to the array "installments" exists.
Thing is, it keeps adding to the database while saying the following message:

(node:3788) UnhandledPromiseRejectionWarning: Installment inexistant!
(node:3788) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside o
f an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the n
ode process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli
.html#cli_unhandled_rejections_mode). (rejection id: 4)
(node:3788) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections tha
t are not handled will terminate the Node.js process with a non-zero exit code.
(node:3788) UnhandledPromiseRejectionWarning: Installment inexistant!
(node:3788) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside o
f an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the n
ode process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli
.html#cli_unhandled_rejections_mode). (rejection id: 7) 

Here’s what I did, I tried to try/catch it but it immediately gave the error…

/Checks if each installment of the tab exists
    check('installments').custom(tab => {
        tab.forEach( async (value) => { //For each value inside the tab
            const installment = await Installment.findById(value) //Try to find the value
                .exec()
                .then(myInstallment => {
                    if (myInstallment === null) { //If it doesn't exist, return an error
                         return Promise.reject('Installment inexistant!');
                    }
                });
        });
        return true; //If all exist, it's all good!
    }),

I hope anyone can find anything, I’ll take anything I can because I’m very confused and the answers I find on StackOverflow don’t seem to apply for this particular case (that or I just don’t understand it…) Thank you so much for reading this! If that helps, here’s the full "Create" function as well:

//Create
router.post('/add',
    body('name').not().isEmpty().withMessage('Le nom ne peut être vide'),
    body('installments').not().isEmpty().withMessage('Le personnage ne peut être relié a aucun jeu'),
    check('name').custom(async value => {
        const character = await Character.find({name: value}).exec();
        if (character.length !== 0) {
            return Promise.reject('Le nom existe déjà');
        }
        return true;
    }),
    //Checks if each installment of the tab exists
    check('installments').custom(tab => {
        tab.forEach( async (value) => { //For each value inside the tab
            const installment = await Installment.findById(value) //Try to find the value
                .exec()
                .then(myInstallment => {
                    if (myInstallment === null) { //If it doesn't exist, return an error
                         return Promise.reject('Installment inexistant!');
                    }
                });
        });
        return true; //If all exist, it's all good!
    }),
    async(req,res,next) => {
        const errors = validationResult(req);
        if (!errors.isEmpty()) {
            res.status(400).send({
                errors: errors.array()
            });
            return;
        }

    const monChar = new Character();
    monChar.name = req.body.name;
    monChar.description = req.body.description;
    monChar.installments = req.body.installments;
    monChar.image = req.body.image;
    await monChar.save();
    res.send(monChar);
});

All the other stuff works well btw! I have my models and all that ^^