Very recently, using SqlDeveloper 4 Data Modeler I tried to pull and import schema object data (i.e tables) to create an Entity Relationship Diagram ERD from an Oracle database.
After carefully going through the required steps, I kicked off the import process. An error popped up immediately on my screen.

The message given:$NullStorageDesign cannot be cast to

The fix is to change the settings for system data types directory in Preferences for Data Modeler in SqlDeveloper.
Make it so that the value for this Preferences setting point to the directory or folder containing types.xml and defaultRDBMSSites.xml files. In my case the path value I plugged in is:

C:program filessqldevelopersqldeveloperextensionsoracle.datamodelertypes

Or it could be where ever it is that you placed the Top folder for SqlDeveloper tool on your computer.
–>  sqldevelopersqldeveloperextensionsoracle.datamodelertypes

To make the Preferences Settings changes, Go to SqlDeveloper Menu. Select Tools -> Preferences -> Data Modeler As shown below.


Click OK button to save and exit Preferences Screen.

You are now ready to Import Table objects into SqlDeveloper’s Data Modeler and create your ERD.

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