[BUG] Creating nodegroups results in context error

**Describe the bug**
When multiple node editors are open in the layout (even shader), context error for `bpy.ops.node.clipboard_copy()` is shown (poll failed).

**To Reproduce**
Steps to reproduce the behavior:
1. Create a simple Sorcar nodetree
2. Drag out node editor layouts in the same window
3. Select some nodes
4. Group the nodes by pressing Ctrl+G (or using the N-Panel operator)

**Expected behavior**
Group is created in the node tree currently active (mouse focus)

**Screenshots**

**Desktop (please complete the following information):**
- OS: Manjaro Linux
- Blender Version: 2.82a
- Sorcar Version: cda1255f06b836ed1f8cbcef5a9d625c58494fba (dev)

**Additional context**
Use `context.space_data.edit_tree` instead of `context.space_data.node_tree`