Layout: Fix error when plugins required for tabification are unavailable

This was causing a crash at startup in some cases.

spyder/plugins/layout/plugin.py

@@ -1029,7 +1029,14 @@ def tabify_helper(plugin, next_to_plugins):
             return False
 
         # Get the actual plugins from their names
-        next_to_plugins = [self.get_plugin(p) for p in next_to_plugins]
+        next_to_plugins = [
+            self.get_plugin(p, error=False) for p in next_to_plugins
+        ]
+
+        # Remove not available plugins from next_to_plugins
+        next_to_plugins = [
+            p for p in next_to_plugins if p is not None
+        ]
 
         if plugin.get_conf('first_time', True):
             # This tabifies external and internal plugins that are loaded for