[Fixed] Sch v6 sub-sheet issues

- When saving a sub-sheet that was edited as a standalone sheet
- In this case we have symbo instances, but they weren't
  processed, so we can't look for filter modifications.

Closes #307

CHANGELOG.md

@@ -53,6 +53,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   (#289 #290)
 - KiCad 5 "assert "lower <= upper" failed in Clamp()" (#304)
 - Missing XYRS information for components with multiple units (#306)
+- Schematic v6: Problems when creating a variant of a sub-sheet that was edited
+  as a standalone sheet (#307)
 
 ### Changed
 - Diff:

kibot/kicad/v6_sch.py

@@ -1733,11 +1733,12 @@ class SchematicV6(Schematic):
             _add_items_list('sheet_instances', self.sheet_instances, sch)
             # Symbol instances
             # Copy potentially modified data from components
-            for s in self.symbol_instances:
-                comp = s.component
-                s.reference = comp.ref
-                s.value = comp.value
-                s.footprint = comp.footprint_lib+':'+comp.footprint if comp.footprint_lib else comp.footprint
+            if base_sheet == self:
+                for s in self.symbol_instances:
+                    comp = s.component
+                    s.reference = comp.ref
+                    s.value = comp.value
+                    s.footprint = comp.footprint_lib+':'+comp.footprint if comp.footprint_lib else comp.footprint
             _add_items_list('symbol_instances', self.symbol_instances, sch)
             logger.debug('Saving schematic: `{}`'.format(fname))
             # Keep a back-up of existing files