Fix Stack:Peek() bug + check for empty stack if appropriate

lib/stack.lua

@@ -1,5 +1,7 @@
+local Position = require("lib.position")
+
 ---@class Stack
----@field elements table[]
+---@field elements Position[]
 local Stack = {}
 Stack.__index = Stack
 
@@ -17,9 +19,10 @@ function Stack:Create()
 end
 
 
----@param element table
+---@param element Position
 function Stack:Push(element)
     if element == nil or element == {} then
+        printError("Failed to push empty element to the stack!")
         return
     end
 
@@ -27,15 +30,25 @@ function Stack:Push(element)
 end
 
 
----@return table
+---@return Position | nil
 function Stack:Pop()
+    if self:Count() == 0 then
+        printError("Failed to pop element from the empty stack!")
+        return nil
+    end
+
     return table.remove(self.elements)
 end
 
 
----@return table
+---@return Position | nil
 function Stack:Peek()
-    return table[#self.elements]
+    if self:Count() == 0 then
+        printError("Failed to peek element from the empty stack!")
+        return nil
+    end
+
+    return self.elements[#self.elements]
 end