for a longer time I have a problem with the attribute table. I create the polygons on the map, I fill up the table with attributes. Then, if I click "save", all the data from the table are being deleted and in all fields of the table a word "error" does appear. After the restart of the program, only that columns containing the number values are being kept. That ones which contain the words are deleted and the word "null" does appear in all the fields. Is somebody able to help me?
Most likely the columns are defined as "numeric".
The problem is: QGIS lets you input any value into attribute table fields. If you input text into numeric fields, everything will look ok until you try to save the edits. Then it will fail to save text into the numeric fields.
There have been discussions about adding warnings to notify users if they input invalid values. They should be added in future versions.
When saving features get a new feature id (from a temporary negative feature id to their "real" feature id). The attribute table keeps track of the features listed in it by id. As soon as they are saved they become unavailable with that id. And the row turned to "ERROR".
With commit d43c89ae that was fixed. QGIS handles the transition as if the original features were removed and the new features were added. That way the old rows disappear and new rows are loaded.