Fix rectangle grid properties (#3082) (#3513)

* Import  for both vertical and horizontal gridlines in

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: Tristan Schulz <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

manim/mobject/geometry/polygram.py

@@ -619,12 +619,16 @@ def __init__(
         super().__init__(UR, UL, DL, DR, color=color, **kwargs)
         self.stretch_to_fit_width(width)
         self.stretch_to_fit_height(height)
+
         v = self.get_vertices()
         self.grid_lines = VGroup()
 
-        if grid_xstep is not None:
+        if grid_xstep or grid_ystep:
             from manim.mobject.geometry.line import Line
 
+            v = self.get_vertices()
+
+        if grid_xstep:
             grid_xstep = abs(grid_xstep)
             count = int(width / grid_xstep)
             grid = VGroup(
@@ -638,7 +642,8 @@ def __init__(
                 )
             )
             self.grid_lines.add(grid)
-        if grid_ystep is not None:
+
+        if grid_ystep:
             grid_ystep = abs(grid_ystep)
             count = int(height / grid_ystep)
             grid = VGroup(