[editor] Only show likert line color picker when line coloring is set

projects/editor/src/app/unit-view/page-view/properties-panel/element-properties.component.html

@@ -336,7 +336,7 @@
           {{'propertiesPanel.lineColoring' | translate }}
         </mat-checkbox>
 
-        <mat-form-field *ngIf="combinedProperties.lineColoringColor"
+        <mat-form-field *ngIf="combinedProperties.lineColoring && combinedProperties.lineColoringColor"
                         appearance="fill" class="mdInput textsingleline">
           <mat-label>{{'propertiesPanel.lineColoringColor' | translate }}</mat-label>
           <input matInput type="color" [value]="combinedProperties.lineColoringColor"