fix selectedOption value

@@ -112,7 +112,7 @@
                           (focus)="key.isFocused = true;"
                           (blur)="key.isFocused = false; inputChanged(source, key)">
                 <mat-option *ngFor="let option of key.settings.selectOptions"
-                            [value]="option.value.toLowerCase() !== 'null' ? option.value : null"
+                            [value]="option.value"
                             [disabled]="key.settings.isEditable === 'readonly'">
                   {{ getCustomTranslationText(option.label ? option.label : option.value) }}
                 </mat-option>

@@ -59,7 +59,7 @@ interface MultipleInputWidgetSettings {
 }
 
 interface MultipleInputWidgetSelectOption {
-  value: string;
+  value: string | null;
   label: string;
 }
 
@@ -253,6 +253,14 @@ export class MultipleInputWidgetComponent extends PageComponent implements OnIni
             }
             // For backward compatibility
 
+            if (dataKey.settings.dataKeyValueType === 'select') {
+              dataKey.settings.selectOptions.forEach((option) => {
+                if (option.value.toLowerCase() === 'null') {
+                  option.value = null;
+                }
+              });
+            }
+
             source.keys.push(dataKey);
           });
         } else {
@@ -452,10 +460,6 @@ export class MultipleInputWidgetComponent extends PageComponent implements OnIni
   }
 
   public getCustomTranslationText(value): string {
-    if (value.toLowerCase() === 'null') {
-      return '';
-    }
-
     return this.utils.customTranslation(value, value);
   }