call: fix conference creation

Change-Id: I540d4d1331b09114005034027bbba090127304ff
diff --git a/src/ChooseContactVC.mm b/src/ChooseContactVC.mm
index b7d3ccd..da9d23d 100644
--- a/src/ChooseContactVC.mm
+++ b/src/ChooseContactVC.mm
@@ -147,8 +147,10 @@
 
 -(void)viewDidLoad {
     [super viewDidLoad];
-    contactsView.style = NSTableViewStylePlain;
-    callsView.style = NSTableViewStylePlain;
+    if (@available(macOS 11.0, *)) {
+        contactsView.style = NSTableViewStylePlain;
+        callsView.style = NSTableViewStylePlain;
+    }
     [self reloadView];
 }