add frontend checks

data/cdata/ui_scripts/MainMenu/__init__.lua

@@ -1,3 +1,7 @@
+if not Engine.InFrontend() then
+    return
+end
+
 require("CampaignMenuButtons")
 require("CPMainMenu")
 require("CPMainMenuButtons")

data/cdata/ui_scripts/Mods/__init__.lua

@@ -1,6 +1,7 @@
+if not Engine.InFrontend() then
+    return
+end
+
 require("ModSelectButton")
 require("ModSelectMenu")
-
-if (Engine.InFrontend()) then
-    require("ModDownload")
-end
+require("ModDownload")

data/cdata/ui_scripts/SystemLinkMenu/__init__.lua

@@ -1,2 +1,6 @@
+if not Engine.InFrontend() then
+    return
+end
+
 require("ServerButtons")
 require("SysLinkBrowser")