[V3] Windows: fix(application): handle error and type assertion in save file dialog (#4284)

* fix(application): handle error and type assertion in save file dialog

---------

Co-authored-by: hkhere <hk@tinyclouds.cn>
Co-authored-by: Atterpac <89053530+atterpac@users.noreply.github.com>
Co-authored-by: Lea Anthony <lea.anthony@gmail.com>

v3/UNRELEASED_CHANGELOG.md

@@ -23,6 +23,7 @@ After processing, the content will be moved to the main changelog and this file
 
 ## Fixed
 <!-- Bug fixes -->
+- Fixed panic when closing or cancelling a `SaveFileDialog` on windows. Fixed in [PR](https://github.com/wailsapp/wails/pull/4284) by @hkhere
 - Fixed HTML level drag and drop on Windows by [@mbaklor](https://github.com/mbaklor) in [#4259](https://github.com/wailsapp/wails/pull/4259)
 
 ## Deprecated

v3/internal/go-common-file-dialog/cfd/vtblCommonFunc.go

@@ -4,10 +4,11 @@ package cfd
 
 import (
 	"fmt"
-	"github.com/go-ole/go-ole"
 	"strings"
 	"syscall"
 	"unsafe"
+
+	"github.com/go-ole/go-ole"
 )
 
 func hresultToError(hr uintptr) error {
@@ -168,7 +169,7 @@ func (vtbl *iFileDialogVtbl) getResultString(objPtr unsafe.Pointer) (string, err
 		return "", err
 	}
 	if shellItem == nil {
-		return "", fmt.Errorf("shellItem is nil")
+		return "", ErrorCancelled
 	}
 	defer shellItem.vtbl.release(unsafe.Pointer(shellItem))
 	return shellItem.vtbl.getDisplayName(unsafe.Pointer(shellItem))

v3/pkg/application/dialogs_windows.go

@@ -196,9 +196,16 @@ func (m *windowSaveFileDialog) show() (chan string, error) {
 		func() (cfd.Dialog, error) {
 			return cfd.NewSaveFileDialog(config)
 		}, false)
+	if err != nil {
+		close(files)
+		return files, err
+	}
 	go func() {
 		defer handlePanic()
-		files <- result.(string)
+		f, ok := result.(string)
+		if ok {
+			files <- f
+		}
 		close(files)
 	}()
 	return files, err