"Function Query.where() called with invalid data. Unsupported field value: undefined" Error in angular with firebase - node.js

Question

I am using angular to create a booking web app using firebase. I am trying to query through the firebase data and filter them to avoid duplicate bookings to be recorded at the same time slots, but I keep getting the error Function Query.where() called with invalid data. Unsupported field value: undefined. I am new to angular and firebase and this is my first project as well. I could really use some help on this.
checkTimeSlot(){
this.formData.date_time = this.formData.bookDate + "_"+ this.formData.timeSlot;
const queryTimeRefUFC = this.afs
.collection("ufc", ref => ref.where("date_time", "==", this.formData.date_time))
.get();
if (queryTimeRefUFC == null){
console.log("true");
return true;
}
else{
console.log("false");
return false;
}}
This is the method in the component.ts where I created the query search. This method is called when the value in the select input changes.
submitBookingUFC() {
this.formData.fullName = this.user.name;
this.formData.userID = this.user.id;
this.formData.date_time = this.formData.bookDate + "_"+ this.formData.timeSlot;
if (this.checkTimeSlot() == true){
alert("The Time Slot you have selected has already been taken. Please select another.");
}
else if (this.checkTimeSlot() == false){
console.log(this.formData.date_time);
this.crudApi.SaveBookingUFC(this.formData);
alert("Record Saved Successfully");
this.ResetFormUFC(); // Reset form when clicked on reset button
} }
This is the method called when the user clicks the submit button.
<div>
<input
type="date"
class="in-bdate"
id="dateUFC"
name="dateUFC"
[min]="today"
max="{{bookMax}}"
value="{{formData.bookDate}}"
[(ngModel)]="formData.bookDate"
#dateUFC="ngModel"
required>
</div>
<div>
<div style="color:red"
*ngIf="dateUFC.errors && (dateUFC.dirty || dateUFC.touched)">
<p *ngIf="dateUFC.errors.required">
<sup>*</sup>Booking Date is required
</p>
</div>
</div>
<div *ngIf="!dateUFC.errors && (dateUFC.dirty || dateUFC.touched)">
<div>
<label for="timesUFC">Time Slot</label>
</div>
<div>
<select
name="timesUFC"
class="in-tslot"
value="{{formData.timeSlot}}"
(change)="checkTimeSlot()"
[(ngModel)]="formData.timeSlot"
#timeSlotUFC="ngModel"
required>
<option *ngFor="let time of timesUFC" >{{time.name}}</option>
</select>
</div>
<div>
<div style="color:red"
*ngIf="(timeSlotUFC.dirty || timeSlotUFC.touched)">
<p *ngIf="timeSlotUFC.errors && timeSlotUFC.errors.required">
<sup>*</sup>Time Slot is required
</p>
<p [ngModel]="timeError" name="error" ngDefaultControl>{{ timeError }}</p>
</div>
</div>
</div>
This is the html code I used to attain the date for the booking and the time of the booking. Using these I created a composite field as date_time using this.formData.date_time = this.formData.bookDate + "_"+ this.formData.timeSlot; to avoid getting duplicate bookings for the same time slot on the same day. The formData here is linked to a class I created for the entire booking array using formData = new UfcData; where UfcData is the name of the class.
export class UfcData {
docID!: string;
userID!: string;
fullName!: string;
contactNo!: string;
timeSlot!: string;
bookDate!: string;
date_time!: string;}
This is the data in the UfcData class.
This is how my Collection and Documents look like
What I need is to acquire the user's input from the form which is acquire from ngModel to the UfcData class and make sure that there aren't any documents in the cloud firestore with the exact same date and time.
*After tweaking the code a little bit the above error does not display. However the if condition in the checkTimeSlot() method always returns true no matter what the option on the select is.
Thanks in advance.