FIX: PrinterFileSystem less thumbnail batch count to 2

Change-Id: If741fa8a394ab510f0bf15fa90cd43c8f14a7c80
Jira: none
(cherry picked from commit bc8f79e2fec6ae568b9f403f21d694659b5dea79)

src/slic3r/GUI/Plater.cpp

@@ -1035,6 +1035,9 @@ void ExtruderGroup::update_ams()
         info1.cans.push_back({});
     }
 
+    if (btn_edit == nullptr)
+        return;
+
     page_num  = (ams_n4 * 2 + ams_n1 + 3) / 4;
     size_t i4 = page_cur * 2;
     size_t i1 = 0;