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我想在更改mat-select时显示或隐藏表单字段。我用来显示或隐藏该过程的以下代码。但是它显示了一个错误：

无法读取未定义的属性“ valueFieldType”。

1）.html文件

    <mat-form-field style="width: 30%">
                 <mat-select formControlName="evaluationRuleField" placeholder="Select Field" [value]="evaluationRuleField" id="evaluationRuleField" name="evaluationRuleField"> 
                  <mat-option *ngFor="let evaluationRuleField of evaluationRuleFields" [value]="evaluationRuleField">{{ evaluationRuleField.viewValue }}</mat-option>
                </mat-select>             
              </mat-form-field>
<!--Start Dynamically Change Field-->
              <mat-form-field class="example-full-width" style="width: 30%" *ngIf = "evaluationRuleField.valueFieldType == 'text'">
                  <input matInput formControlName="evaluationRuleValue" placeholder="Value" [ngModel]="evaluationRuleValue" id="evaluationRuleValue" name="evaluationRuleValue" required>
              </mat-form-field>
              <mat-form-field class="example-full-width" style="width: 30%" *ngIf = "evaluationRuleField.valueFieldType == 'dropdwon'">
                <mat-select formControlName="evaluationRuleField" placeholder="Select Field" [(value)]="ruleValueFields" id="evaluationRuleField" name="evaluationRuleField" (change)="getValue()"> 
                  <mat-option *ngFor="let ruleValueField of ruleValueFields" [value]="ruleValueField.value">{{ ruleValueField.viewValue }}</mat-option>
                </mat-select>   
              </mat-form-field>
              <mat-form-field class="example-full-width" style="width: 30%" *ngIf = "evaluationRuleField.valueFieldType …