Disable Start button until mission is selected.

The mission selection map starts with nothing selected so the "Start"
button does nothing on click.
Disable it until something gets selected for better UX.

libs/s25main/desktops/dskCampaignMissionSelection.cpp

@@ -67,13 +67,15 @@ dskCampaignMissionSelection::dskCampaignMissionSelection(CreateServerInfo csi, c
     if(campaign_->selectionMapData)
     {
         constexpr Extent buttonSize(200, buttonHeight);
-        AddTextButton(ID_btStart, DrawPoint(300, 530), buttonSize, TextureColor::Red1, _("Start"), NormalFont);
+        auto* btStart =
+          AddTextButton(ID_btStart, DrawPoint(300, 530), buttonSize, TextureColor::Red1, _("Start"), NormalFont);
         AddTextButton(ID_btBack, DrawPoint(300, 530 + buttonSize.y + spacingBetweenButtons), buttonSize,
                       TextureColor::Red1, _("Back"), NormalFont);
 
         auto* mapSelection =
           AddMapSelection(ID_MapSelection, DrawPoint(0, 0), Extent(800, 508), *campaign_->selectionMapData);
         mapSelection->setMissionsStatus(std::vector<MissionStatus>(campaign_->getNumMaps(), {true, true}));
+        btStart->SetEnabled(static_cast<bool>(mapSelection->getSelection()));
     } else
     {
         constexpr Extent buttonSize(22, buttonHeight);
@@ -194,6 +196,11 @@ void dskCampaignMissionSelection::Msg_ButtonClick(unsigned ctrl_id)
         const auto selectedMission = GetCtrl<ctrlMapSelection>(ID_MapSelection)->getSelection();
         if(selectedMission)
             StartServer(*selectedMission);
+    } else if(ctrl_id == ID_MapSelection)
+    {
+        auto* btStart = GetCtrl<ctrlButton>(ID_btStart);
+        const auto* mapSelection = GetCtrl<ctrlMapSelection>(ID_MapSelection);
+        btStart->SetEnabled(static_cast<bool>(mapSelection->getSelection()));
     } else if(ctrl_id >= ID_btFirstPage && ctrl_id <= ID_btLastPage)
     {
         switch(ctrl_id)