Fix a lock that is held too long

I can only guess, but I think this was a typo (or a copy-paste-o) when this code
was written. It was introduced in 55af07a1bb, and I think the defer was kept by
accident; if it had been on purpose, then the statement would have been put
right after the Lock call.

pkg/gui/context.go

@@ -51,7 +51,7 @@ func (self *ContextMgr) Replace(c types.Context) error {
 		self.ContextStack = append(self.ContextStack[0:len(self.ContextStack)-1], c)
 	}
 
-	defer self.Unlock()
+	self.Unlock()
 
 	return self.Activate(c, types.OnFocusOpts{})
 }