[partition] Be defensive against no device-device-available.

Scenario is this: you have no suitable installation devices on
your system (everything is mounted, or HDD has died), click through
to partition page, where you have all the buttons available, but no
devices in the list. The following actions then cause a crash:

 - clicking "back"
 - clicking any button

Prevent that:
 - you can click "back", but if there is no device selected
   nothing happens to the device state (no nullptr deref,
   and no crash)
 - button code is now more resilient to this scenario
 - buttons are hidden until a device is available, so you
   can't even click on them to trigger the code.

src/modules/partition/gui/ChoicePage.cpp

@@ -300,6 +300,16 @@ ChoicePage::selectedDevice()
 }
 
 
+void
+ChoicePage::hideButtons()
+{
+    m_eraseButton->hide();
+    m_replaceButton->hide();
+    m_alongsideButton->hide();
+    m_somethingElseButton->hide();
+}
+
+
 /**
  * @brief ChoicePage::applyDeviceChoice handler for the selected event of the device
  *      picker. Calls ChoicePage::selectedDevice() to get the current Device*, then
@@ -311,7 +321,10 @@ void
 ChoicePage::applyDeviceChoice()
 {
     if ( !selectedDevice() )
+    {
+        hideButtons();
         return;
+    }
 
     if ( m_core->isDirty() )
     {
@@ -342,11 +355,14 @@ ChoicePage::continueApplyDeviceChoice()
     // applyDeviceChoice() will be called again momentarily as soon as we handle the
     // PartitionCoreModule::reverted signal.
     if ( !currd )
+    {
+        hideButtons();
         return;
+    }
 
     updateDeviceStatePreview();
-    // Preview setup done. Now we show/hide choices as needed.
 
+    // Preview setup done. Now we show/hide choices as needed.
     setupActions();
 
     m_lastSelectedDeviceIndex = m_drivesCombo->currentIndex();
@@ -562,7 +578,11 @@ ChoicePage::onLeave()
         {
             if ( m_bootloaderComboBox.isNull() )
             {
-                m_core->setBootLoaderInstallPath( selectedDevice()->deviceNode() );
+                auto d_p = selectedDevice();
+                if ( d_p )
+                    m_core->setBootLoaderInstallPath( d_p->deviceNode() );
+                else
+                    cDebug() << "WARNING: No device selected for bootloader.";
             }
             else
             {
@@ -1156,6 +1176,9 @@ ChoicePage::setupActions()
     else
         m_deviceInfoWidget->setPartitionTableType( PartitionTable::unknownTableType );
 
+    // Manual partitioning is always a possibility
+    m_somethingElseButton->show();
+
     bool atLeastOneCanBeResized = false;
     bool atLeastOneCanBeReplaced = false;
     bool atLeastOneIsMounted = false;  // Suppress 'erase' if so
@@ -1332,18 +1355,16 @@ ChoicePage::updateNextEnabled()
 {
     bool enabled = false;
 
+    auto sm_p = m_beforePartitionBarsView ? m_beforePartitionBarsView->selectionModel() : nullptr;
+
     switch ( m_choice )
     {
     case NoChoice:
         enabled = false;
         break;
     case Replace:
-        enabled = m_beforePartitionBarsView->selectionModel()->
-                  currentIndex().isValid();
-        break;
     case Alongside:
-        enabled = m_beforePartitionBarsView->selectionModel()->
-                  currentIndex().isValid();
+        enabled = sm_p && sm_p->currentIndex().isValid();
         break;
     case Erase:
     case Manual:

src/modules/partition/gui/ChoicePage.h

@@ -113,8 +113,12 @@ private:
     void setupChoices();
     QComboBox* createBootloaderComboBox( QWidget* parentButton );
     Device* selectedDevice();
-    void applyDeviceChoice();
-    void continueApplyDeviceChoice();
+
+    /* Change the UI depending on the device selected. */
+    void hideButtons();  // Hide everything when no device
+    void applyDeviceChoice();  // Start scanning new device
+    void continueApplyDeviceChoice();  // .. called after scan
+
     void updateDeviceStatePreview();
     void updateActionChoicePreview( ChoicePage::Choice choice );
     void setupActions();