8128: GtkDialog mapped without a transient parent

The set of fixes was actually triggered by comments in
https://gramps-project.org/bugs/view.php?id=9122#c45895
https://gramps-project.org/bugs/view.php?id=9122#c45921

The project is ongoing, since it's needed for our Windows users.

gramps/gui/user.py

@@ -123,7 +123,10 @@ class User(user.User):
         :type warning: str
         :returns: none
         """
-        WarningDialog(title, warning)
+        if self.uistate:
+            WarningDialog(title, warning, parent=self.uistate.window)
+        else:
+            WarningDialog(title, warning)
     
     def notify_error(self, title, error=""):
         """
@@ -137,6 +140,8 @@ class User(user.User):
         """
         if self.error_function:
             self.error_function(title, error)
+        elif self.uistate:
+            ErrorDialog(title, error, parent=self.uistate.window)
         else:
             ErrorDialog(title, error)
 
@@ -148,7 +153,10 @@ class User(user.User):
         :type error: str
         :returns: none
         """
-        DBErrorDialog(error)
+        if self.uistate:
+            DBErrorDialog(error, parent=self.uistate.window)
+        else:
+            DBErrorDialog(error)
 
     def info(self, msg1, infotext, parent=None, monospaced=False):
         """