Hi, why is my preview crashing when adding category to a transaction with exampleTransaction.category = exampleCategory?

Is there any good source where can I learn more about SwiftData and Previews? I'm really lost in Apple docs.

#Preview {
    do {
        let config = ModelConfiguration(isStoredInMemoryOnly: true)
        let container = try ModelContainer(for: Transaction.self, configurations: config)

        let exampleCategory = Category(title: "Salary", icon: "banknote", color: "#00ff00", budgetType: BudgetType.fixed)
        var exampleTransaction = Transaction(amount: 2836.76, date: .now, notes: "", isIncome: true)

        exampleTransaction.category = exampleCategory

        return TransactionRowView(transaction: exampleTransaction)
            .modelContainer(container)
    } catch {
        fatalError("Failed to create model container.")
    }
}

Category model:

import Foundation
import SwiftData

enum BudgetType: String, Codable {
    case fixed
    case flexible
    case savings
}

@Model
final class Category {
    var title: String
    var icon: String
    var color: String
    var budgetType: BudgetType
    var transactions = [Transaction]()

    init(title: String, icon: String, color: String, budgetType: BudgetType) {
        self.title = title
        self.icon = icon
        self.color = color
        self.budgetType = budgetType
    }
}

Transaction model:

import Foundation
import SwiftData

@Model
final class Transaction {
    var amount: Double
    var date: Date
    var notes: String
    var isIncome: Bool
    var category: Category?

    init(amount: Double, date: Date, notes: String, isIncome: Bool) {
        self.amount = amount
        self.date = date
        self.notes = notes
        self.isIncome = isIncome
    }
}