Enhance TemplatePreview with Clear Iframe Contents Functionality

- Added a new function to clear iframe contents, improving resource management and preventing stale data display.
- Integrated the clearIframeContents function into the PDF preview type change handler, ensuring a clean slate when switching to PDF previews.
- Updated state management for iframe size and preview content to enhance user experience and maintain consistency.

src/components/Dashboard/common/TemplatePreview.jsx

@@ -171,6 +171,19 @@ const TemplatePreview = ({
     setPreviewContentHTML(html)
   }
 
+  const clearIframeContents = useCallback(() => {
+    iframeSizeObserverRef.current?.disconnect()
+    iframeSizeObserverRef.current = null
+
+    const iframe = iframeRef.current
+    if (iframe) {
+      iframe.srcdoc = ''
+    }
+
+    setPreviewContentHTML('')
+    setIframeSize({ width: '100%', height: 'auto' })
+  }, [])
+
   const stopRenderProgressListener = useCallback(() => {
     if (typeof renderUnsubscribeRef.current === 'function') {
       renderUnsubscribeRef.current()
@@ -541,10 +554,11 @@ const TemplatePreview = ({
             disabled={loading || reloadLoading}
             value={previewType}
             onChange={(value) => {
-              setPreviewType(value)
               if (value == 'PDF') {
+                clearIframeContents()
                 setPDFBlob(null)
               }
+              setPreviewType(value)
             }}
           />
         ) : null}