Error: Transaction reverted without a reason string

[monzzzz]

`const { deployments, ethers, getNamedAccounts } = require("hardhat")
const { assert, expect } = require("chai")
describe("FundMe", async function () {
let fundMe
let deployer
let MockV3Aggregator
const sendValue = ethers.utils.parseEther("1") //1 eth
beforeEach(async function () {
//deploy contract Fundme contract
//using Hardhat deploy
// const accounts = await ethers.getSigners()
// const accountoZero = accounts[0]
deployer = (await getNamedAccounts()).deployer
await deployments.fixture(["all"])
fundMe = await ethers.getContract("FundMe", deployer)
MockV3Aggregator = await ethers.getContract(
"MockV3Aggregator",
deployer
)
})
describe("constructor", async function () {
it("sets the aggregator addresses correctly", async function () {
const response = await fundMe.priceFeed()
assert.equal(response, MockV3Aggregator.address)
})
})

describe("fund", async function () {
it("Fails if you don't send enough ETH", async function () {
await expect(fundMe.fund()).to.be.revertedWith("Didn't send enough")
})
it("updated the amount funded data structure", async function () {
await fundMe.fund({ value: sendValue })
const response = await fundMe.addressToAmountFunded(deployer)
assert.equal(response.toString(), sendValue.toString())
})
it("Adds funder to array of funders", async function () {
await fundMe.fund({ value: sendValue })
const funder = await fundMe.funders(0)
assert.equal(funder, deployer)
})
})
describe("withdraw", async function () {
beforeEach(async function () {
await fundMe.fund({ value: sendValue })
})

it("withdraw ETH from a single founder", async function () {
    //Arrange
    const startingFundMeBalance = await fundMe.provider.getBalance(
        //get balance of any contract which is fundMe.address
        fundMe.address
    )
    const startingDeployerBalance = await fundMe.provider.getBalance(
        deployer
    )
    //Act
    const transactionResponse = await fundMe.withdraw()
    const transactionReceipt = await transactionResponse.wait(1)
    const { gasUsed, effectiveGasPrice } = transactionReceipt
    const gasCost = gasUsed.mul(effectiveGasPrice)
    const endingFundMeBalance = await fundMe.provider.getBalance(
        fundMe.address
    )
    const endingDeployerBalance = await fundMe.provider.getBalance(
        deployer
    )
    //gasCost
    //Assert
    assert.equal(endingFundMeBalance, 0)
    assert.equal(
        startingFundMeBalance.add(startingDeployerBalance).toString(),
        endingDeployerBalance.add(gasCost).toString()
    )
})

})
})
`
I got this error

FundMe
constructor
✔ sets the aggregator addresses correctly
fund
✔ Fails if you don't send enough ETH (52ms)
✔ updated the amount funded data structure
✔ Adds funder to array of funders
withdraw

1. withdraw ETH from a single founder

4 passing (441ms)
1 failing

FundMe
withdraw
withdraw ETH from a single founder:
Error: Transaction reverted without a reason string
at FundMe.withdraw (contracts/FundMe.sol:86)
at HardhatNode._mineBlockWithPendingTxs (node_modules/hardhat/src/internal/hardhat-network/provider/node.ts:1772:23)
at HardhatNode.mineBlock (node_modules/hardhat/src/internal/hardhat-network/provider/node.ts:466:16)
at EthModule._sendTransactionAndReturnHash (node_modules/hardhat/src/internal/hardhat-network/provider/modules/eth.ts:1496:18)
at HardhatNetworkProvider.request (node_modules/hardhat/src/internal/hardhat-network/provider/provider.ts:118:18)
at EthersProviderWrapper.send (node_modules/@nomiclabs/hardhat-ethers/src/internal/ethers-provider-wrapper.ts:13:20)
error Command failed with exit code 1.

how can I do?

[monzzzz]

I deleted revert() in fundMe file, and I got it.