Fix simple null pointer exception (#82)

* Update math.dart * Update selectable.dart

Author: fzyzcjy

lib/src/widgets/math.dart

@@ -187,7 +187,7 @@ class Math extends StatelessWidget {
  options = MathOptions(
  style: mathStyle,
  fontSize: effectiveTextStyle.fontSize! * textScaleFactor,
- mathFontOptions: effectiveTextStyle.fontWeight != FontWeight.normal
+ mathFontOptions: effectiveTextStyle.fontWeight != FontWeight.normal && effectiveTextStyle.fontWeight != null
  ? FontOptions(fontWeight: effectiveTextStyle.fontWeight!)
  : null,
  logicalPpi: logicalPpi,

lib/src/widgets/selectable.dart

@@ -242,7 +242,7 @@ class SelectableMath extends StatelessWidget {
  MathOptions(
  style: mathStyle,
  fontSize: effectiveTextStyle.fontSize! * textScaleFactor,
- mathFontOptions: effectiveTextStyle.fontWeight != FontWeight.normal
+ mathFontOptions: effectiveTextStyle.fontWeight != FontWeight.normal && effectiveTextStyle.fontWeight != null
  ? FontOptions(fontWeight: effectiveTextStyle.fontWeight!)
  : null,
  logicalPpi: logicalPpi,