Fix crash on trying to start in folder-less workspace (#34)

Resolves #29

plugin.py

@@ -1,9 +1,13 @@
 import os
 import sublime
+from LSP.plugin import ClientConfig
+from LSP.plugin import uri_to_filename
+from LSP.plugin import WorkspaceFolder
 from LSP.plugin.core.protocol import Point
-from LSP.plugin.core.url import uri_to_filename
+from LSP.plugin.core.types import List, Optional
 from LSP.plugin.core.views import point_to_offset
 from lsp_utils import NpmClientHandler
+from lsp_utils import request_handler
 
 
 def plugin_loaded():
@@ -25,13 +29,21 @@ class LspTypescriptPlugin(NpmClientHandler):
     def install_in_cache(cls) -> bool:
         return False
 
-    def on_ready(self, api) -> None:
-        api.on_request('_typescript.rename', self._on_typescript_rename)
-
-    def _on_typescript_rename(self, textDocumentPositionParams, respond):
-        view = sublime.active_window().open_file(
-            uri_to_filename(textDocumentPositionParams['textDocument']['uri'])
-        )
+    @classmethod
+    def is_allowed_to_start(
+        cls,
+        window: sublime.Window,
+        initiating_view: Optional[sublime.View] = None,
+        workspace_folders: Optional[List[WorkspaceFolder]] = None,
+        configuration: Optional[ClientConfig] = None
+    ) -> Optional[str]:
+        if not workspace_folders:
+            return 'This server only works when the window workspace includes some folders!'
+
+    @request_handler('_typescript.rename')
+    def on_typescript_rename(self, textDocumentPositionParams, respond):
+        filename = uri_to_filename(textDocumentPositionParams['textDocument']['uri'])
+        view = sublime.active_window().open_file(filename)
 
         if view:
             lsp_point = Point.from_lsp(textDocumentPositionParams['position'])