additional nnbd fixes (#227)

diff --git a/lib/src/vector_math/aabb2.dart b/lib/src/vector_math/aabb2.dart
index 2222c3d..a20a8c3 100644
--- a/lib/src/vector_math/aabb2.dart
+++ b/lib/src/vector_math/aabb2.dart
@@ -76,8 +76,8 @@
     _max.setFrom(other._max);
   }
 
-  static late final _center = Vector2.zero();
-  static late final _halfExtents = Vector2.zero();
+  static final _center = Vector2.zero();
+  static final _halfExtents = Vector2.zero();
   void _updateCenterAndHalfExtents() =>
       copyCenterAndHalfExtents(_center, _halfExtents);
 
diff --git a/lib/src/vector_math/aabb3.dart b/lib/src/vector_math/aabb3.dart
index 6c58e14..23b6d66 100644
--- a/lib/src/vector_math/aabb3.dart
+++ b/lib/src/vector_math/aabb3.dart
@@ -203,8 +203,8 @@
     _max.setFrom(other._max);
   }
 
-  static late final _center = Vector3.zero();
-  static late final _halfExtents = Vector3.zero();
+  static final _center = Vector3.zero();
+  static final _halfExtents = Vector3.zero();
   void _updateCenterAndHalfExtents() =>
       copyCenterAndHalfExtents(_center, _halfExtents);
 
diff --git a/lib/src/vector_math_64/aabb2.dart b/lib/src/vector_math_64/aabb2.dart
index 88aa1e6..b3ac51d 100644
--- a/lib/src/vector_math_64/aabb2.dart
+++ b/lib/src/vector_math_64/aabb2.dart
@@ -76,8 +76,8 @@
     _max.setFrom(other._max);
   }
 
-  static late final _center = Vector2.zero();
-  static late final _halfExtents = Vector2.zero();
+  static final _center = Vector2.zero();
+  static final _halfExtents = Vector2.zero();
   void _updateCenterAndHalfExtents() =>
       copyCenterAndHalfExtents(_center, _halfExtents);
 
diff --git a/lib/src/vector_math_64/aabb3.dart b/lib/src/vector_math_64/aabb3.dart
index 96e8da2..ece6138 100644
--- a/lib/src/vector_math_64/aabb3.dart
+++ b/lib/src/vector_math_64/aabb3.dart
@@ -203,8 +203,8 @@
     _max.setFrom(other._max);
   }
 
-  static late final _center = Vector3.zero();
-  static late final _halfExtents = Vector3.zero();
+  static final _center = Vector3.zero();
+  static final _halfExtents = Vector3.zero();
   void _updateCenterAndHalfExtents() =>
       copyCenterAndHalfExtents(_center, _halfExtents);
 
diff --git a/lib/src/vector_math_geometry/generators/circle_generator.dart b/lib/src/vector_math_geometry/generators/circle_generator.dart
index 582d123..a6d14f4 100644
--- a/lib/src/vector_math_geometry/generators/circle_generator.dart
+++ b/lib/src/vector_math_geometry/generators/circle_generator.dart
@@ -5,10 +5,10 @@
 part of vector_math_geometry;
 
 class CircleGenerator extends GeometryGenerator {
-  late final double _radius;
-  late final int _segments;
-  late final double _thetaStart;
-  late final double _thetaLength;
+  late double _radius;
+  late int _segments;
+  late double _thetaStart;
+  late double _thetaLength;
 
   @override
   int get vertexCount => _segments + 2;
diff --git a/lib/src/vector_math_geometry/generators/cube_generator.dart b/lib/src/vector_math_geometry/generators/cube_generator.dart
index a653ee4..2309eb5 100644
--- a/lib/src/vector_math_geometry/generators/cube_generator.dart
+++ b/lib/src/vector_math_geometry/generators/cube_generator.dart
@@ -5,9 +5,9 @@
 part of vector_math_geometry;
 
 class CubeGenerator extends GeometryGenerator {
-  late final double _width;
-  late final double _height;
-  late final double _depth;
+  late double _width;
+  late double _height;
+  late double _depth;
 
   @override
   int get vertexCount => 24;
diff --git a/lib/src/vector_math_geometry/generators/ring_generator.dart b/lib/src/vector_math_geometry/generators/ring_generator.dart
index e90f1fa..c83cd6b 100644
--- a/lib/src/vector_math_geometry/generators/ring_generator.dart
+++ b/lib/src/vector_math_geometry/generators/ring_generator.dart
@@ -5,12 +5,12 @@
 part of vector_math_geometry;
 
 class RingGenerator extends GeometryGenerator {
-  late final double _innerRadius;
-  late final double _outerRadius;
-  late final int _segments;
-  late final double _thetaStart;
-  late final double _thetaLength;
-  late final bool _stripTextureCoordinates;
+  late double _innerRadius;
+  late double _outerRadius;
+  late int _segments;
+  late double _thetaStart;
+  late double _thetaLength;
+  late bool _stripTextureCoordinates;
 
   @override
   int get vertexCount => (_segments + 1) * 2;