Fix #1348: Topic Size issue fixed

[rt4914]

Explanation

Fixes #1348

Currently in develop, TopicInfoFragment displays 0 Bytes as topic size. This is fixed in this PR.
Note that the topic size can be calculated only for those topics which are coming from json files and therefore First Topic and Second Topic will still show topic size as 0 bytes.

Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The PR explanation includes the words "Fixes #bugnum: ..." (or "Fixes part of #bugnum" if the PR only partially fixes an issue).
• The PR follows the style guide.
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• The PR is made from a branch that's not called "develop".
• The PR is made from a branch that is up-to-date with "develop".
• The PR's branch is based on "develop" and not on any other branch.
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[veena14cs]

LGTM.