fix: typecast error in dashboards.loadDashboardData

[paulo242]

type 'double' is not a subtype of type 'int?' in type cast

When loading the dashboard from json, the fromMap of property gridBackgroundParams somethimes returns a type cast error when a double values is represented as an int in the json saved data.

Steps To Reproduce

1. Add elements changing gridBackgroundParams properties
2. Save a dashboard using toJson() or prettyJson()
3. Load a dashboard using loadDashboardData

Expected Behavior

Dashboard saving data in a proper format or loading it as int or double?