diff --git a/amcr_viewer/amcr_dialog.py b/amcr_viewer/amcr_dialog.py
index a91cc0a..2c3a154 100644
--- a/amcr_viewer/amcr_dialog.py
+++ b/amcr_viewer/amcr_dialog.py
@@ -7,6 +7,7 @@ from qgis.PyQt.QtWidgets import (QDialog, QVBoxLayout,
 from qgis.PyQt.QtCore import Qt, QSettings
 from qgis.core import (QgsTask, QgsApplication,
                        QgsMessageLog, Qgis, QgsAuthMethodConfig)
+from qgis.utils import iface
 from .amcr_codelists import (OBDOBI, TYP_AKCE, KRAJE, AREAL, ORGANIZACE,
                              OKRESY, KATASTRY, VEDOUCI, PIAN_PRESNOST,
                              TYP_LOKALITY, DRUH_LOKALITY, JISTOTA,
@@ -14,6 +15,12 @@ from .amcr_codelists import (OBDOBI, TYP_AKCE, KRAJE, AREAL, ORGANIZACE,
                              download_heslare, refresh_globals)
 
 
+# Keep Python references to running tasks. QgsTaskManager only holds the
+# C++ object; without a Python-side reference the wrapper can be garbage
+# collected before the task finishes, which crashes QGIS.
+_ACTIVE_TASKS = []
+
+
 class UpdateCodelistsTask(QgsTask):
     def __init__(self, description):
         super().__init__(description, QgsTask.CanCancel)
@@ -370,21 +377,38 @@ class AmcrFilterDialog(QDialog):
         return row_widget
 
     def action_update_heslare(self):
-        # Create the task instance
+        # Create the task instance and keep a reference so the Python
+        # wrapper survives until the task finishes
         task = UpdateCodelistsTask("Aktualizace heslářů AMČR")
+        _ACTIVE_TASKS.append(task)
 
-        # Re-enable the button regardless of the outcome
-        task.taskCompleted.connect(lambda: self.btn_update.setEnabled(True))
-        task.taskTerminated.connect(lambda: self.btn_update.setEnabled(True))
+        # Prevent parallel downloads overwriting heslar.csv
+        self.btn_update.setEnabled(False)
 
-        task.taskCompleted.connect(lambda: QMessageBox.information(
-            self,
-            "Hotovo",
-            "Hesláře byly úspěšně aktualizovány."
-        ))
+        # Message boxes are parented to the main window, not to this dialog –
+        # the dialog may already be closed (and its C++ object deleted)
+        # by the time the minute-long task finishes.
+        parent_win = iface.mainWindow() if iface else None
+
+        def _cleanup():
+            if task in _ACTIVE_TASKS:
+                _ACTIVE_TASKS.remove(task)
+            try:
+                self.btn_update.setEnabled(True)
+            except RuntimeError:
+                pass  # dialog already closed
+
+        def on_completed():
+            _cleanup()
+            QMessageBox.information(
+                parent_win,
+                "Hotovo",
+                "Hesláře byly úspěšně aktualizovány."
+            )
 
         # Show the exact error if the task fails
         def on_error():
+            _cleanup()
             if task.exception:
                 # This will show exactly what went wrong (e.g. PermissionError)
                 msg = (
@@ -393,8 +417,9 @@ class AmcrFilterDialog(QDialog):
                 )
             else:
                 msg = "Aktualizace byla zrušena uživatelem."
-            QMessageBox.warning(self, "Chyba / Zrušeno", msg)
+            QMessageBox.warning(parent_win, "Chyba / Zrušeno", msg)
 
+        task.taskCompleted.connect(on_completed)
         task.taskTerminated.connect(on_error)
 
         QgsApplication.taskManager().addTask(task)
diff --git a/amcr_viewer/amcr_viewer.py b/amcr_viewer/amcr_viewer.py
index ac1e44c..e28de5b 100644
--- a/amcr_viewer/amcr_viewer.py
+++ b/amcr_viewer/amcr_viewer.py
@@ -6,7 +6,6 @@ from qgis.core import Qgis
 
 from .amcr_tools import load_amcr_data, login_to_api
 from .amcr_dialog import AmcrFilterDialog, LoginDialog
-from .resources import *
 import os.path
 
 
@@ -24,8 +23,9 @@ class AmcrViewer:
         self.iface = iface
         self.plugin_dir = os.path.dirname(__file__)
 
-        # Determine the user's locale to load appropriate translation files
-        locale = QSettings().value('locale/userLocale')[0:2]
+        # Determine the user's locale to load appropriate translation files.
+        # The setting may be missing (None) on a fresh QGIS install.
+        locale = str(QSettings().value('locale/userLocale') or 'en')[0:2]
         locale_path = os.path.join(
             self.plugin_dir,
             'i18n',