Don't try to load the shader code if the file couldn't be opened

src/Filesystem.gd

@@ -61,6 +61,7 @@ func save_result(path: String):
 func load_shader(path: String):
 	print("Load ", path)
 	var file = FileAccess.open(path, FileAccess.READ)
+	if file != null:
 		self.shader_code = file.get_as_text()
 		if "/" in path: # update current working directory
 			self.cwd = path.substr(0, path.rfind("/"))