Your firewall has a version number null which is not supported by ASDM 6.2(5). I received this error when trying to run asdm on my asa 5505. I upgraded image and asdm trying different versions. I used many different versions of java all to no avail. In order to fix this issue you must have the same authentication for ssh and http in your AAA commands. For example, I had ssh to use TACACS but http authentication was set for LOCAL. When I changed AAA for http to TACACS the problem went away and I was able to use asdm.