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* [X] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [X] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [X] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**
```
10.0.0-nightly.20200327 < GTK dialog does open, crashes after dialog closes
10.0.0-nightly.20200326 < GTK dialog doesn't open, crashes after multiple button clicks
10.0.0-nightly.20200325 < GTK dialog doesn't open, crashes after multiple button clicks
10.0.0-nightly.20200324 < GTK dialog doesn't open, crashes after multiple button clicks
10.0.0-nightly.20200323 < GTK dialog doesn't open, crashes after multiple button clicks
10.0.0-nightly.20200320 < GTK dialog doesn't open, crashes after multiple button clicks
10.0.0-nightly.20200317 < GTK dialog doesn't open, crashes after multiple button clicks
10.0.0-nightly.20200310 < Working as expected
```
* **Operating System:**
Ubuntu 18.04 x64
* **Last Known Working Electron version:**


### Expected Behavior
<!-- A clear and concise description of what you expected to happen. -->
I expect to be able to use the GTK FileChooser to upload/download files.

### Actual Behavior
<!-- A clear and concise description of what actually happened. -->
From nightly version `10.0.0-nightly.20200317` the electron window closes without showing the file GTK FileChooser and reports a SEGFAULT in the terminal window. 

From nightly version `10.0.0-nightly.20200327` the GTK FileChooser opens, but after opening a file or cancelling, the electron window exits and reports a SEGFAULT in the terminal window.

### To Reproduce
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`electron  http://jsfiddle.net/lovlka/N4Jxk/`

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### Screenshots
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### Additional Information

shell/browser/ui/file_dialog_gtk.cc
gchar* filename =
      gtk_file_chooser_get_preview_filename(GTK_FILE_CHOOSER(chooser));
  if (!filename) {
    gtk_file_chooser_set_preview_widget_active(GTK_FILE_CHOOSER(chooser),
                                               FALSE);
    return;
  }
  
  
  struct stat stat_buf;
  if (stat(filename, &stat_buf) != 0 || !S_ISREG(stat_buf.st_mode)) {
    g_free(filename);
    gtk_file_chooser_set_preview_widget_active(GTK_FILE_CHOOSER(chooser),
                                               FALSE);
    return;
  }
  
  GdkPixbuf* pixbuf = gdk_pixbuf_new_from_file_at_size(filename, kPreviewWidth,
                                                       kPreviewHeight, nullptr);
  g_free(filename);
  if (pixbuf) {
    gtk_image_set_from_pixbuf(GTK_IMAGE(preview_), pixbuf);
    g_object_unref(pixbuf);
  }
  gtk_file_chooser_set_preview_widget_active(GTK_FILE_CHOOSER(chooser),
                                             pixbuf ? TRUE : FALSE);
}
}  
bool ShowOpenDialogSync(const DialogSettings& settings,